The Colley or Cowley family had lived in that part of Kildare since the time of Wellington's ancestor, Sir Henry Colley or Cowley, who died before 2 October 1584. Sir Henry in his lifetime possessed Carbury Castle, in north-west Kildare, starting with a 21-year lease in 1554.

Wellesley was born into an aristocratic Anglo-Irish family in Ireland as The Hon. Arthur Wesley, the third of five surviving sons (fourth otherwise) to Anne and Garret Wesley, 1st Earl of Mornington. His mother was the eldest daughter of The 1st Viscount Dungannon. As such, he belonged to the Protestant Ascendancy. His biographers mostly follow the same contemporary newspaper evidence in saying that he was born on 1 May 1769, the day before he was baptised. His birthplace is uncertain. He was most likely born at his parents' townhouse, 24 Upper Merrion Street, Dublin, now the Merrion Hotel. But his mother Anne, Countess of Mornington, recalled in 1815 that he had been born at 6 Merrion Street, Dublin. Other places have been put forward as the location of his birth, including Mornington House (the house next door on Upper Merrion), as his father had asserted; the Dublin packet boat; and the mansion in the family estate of Athy (consumed in the fires of 1916), as the Duke apparently put on his 1851 census return.

He went to the diocesan school in Trim when at Dangan, Mr Whyte's Academy when in Dublin, and Brown's School in Chelsea when in London. He then enrolled at Eton College, where he studied from 1781 to 1784. His loneliness there caused him to hate it, and makes it highly unlikely that he actually said "The Battle of Waterloo was won on the playing fields of Eton", a quotation which is often attributed to him. Moreover, Eton had no playing fields at the time. In 1785, a lack of success at Eton, combined with a shortage of family funds due to his father's death, forced the young Wellesley and his mother to move to Brussels. Until his early twenties, Arthur showed little sign of distinction and his mother grew increasingly concerned at his idleness, stating, "I don't know what I shall do with my awkward son Arthur."

A year later, Arthur enrolled in the French Royal Academy of Equitation in Angers, where he progressed significantly, becoming a good horseman and learning French, which later proved very useful. Upon returning to England in late 1786, he astonished his mother with his improvement.

Despite his new promise, he had yet to find a job and his family was still short of money, so upon the advice of his mother, his brother Richard asked his friend the Duke of Rutland (then Lord Lieutenant of Ireland) to consider Arthur for a commission in the Army. Soon afterward, on 7 March 1787, he was gazetted ensign in the 73rd Regiment of Foot. In October, with the assistance of his brother, he was assigned as aide-de-camp, on ten shillings a day (twice his pay as an ensign), to the new Lord Lieutenant of Ireland, Lord Buckingham. He was also transferred to the new 76th Regiment forming in Ireland and on Christmas Day, 1787, was promoted to lieutenant. During his time in Dublin his duties were mainly social; attending balls, entertaining guests and providing advice to Buckingham. While in Ireland, he overextended himself in borrowing due to his occasional gambling, but in his defence stated that "I have often known what it was to be in want of money, but I have never got helplessly into debt".

On 23 January 1788, he transferred into the 41st Regiment of Foot, then again on 25 June 1789, still a lieutenant, he transferred to the 12th (Prince of Wales's) Regiment of (Light) Dragoons and, according to military Historian Richard Holmes, he also dipped a reluctant toe into politics. Shortly before the general election of 1789, he went to the rotten borough of Trim to speak against the granting of the title "Freeman" of Dublin to the parliamentary leader of the Irish Patriot Party, Henry Grattan. Succeeding, he was later nominated and duly elected as a Member of Parliament (MP) for Trim in the Irish House of Commons. Because of the limited suffrage at the time, he sat in a parliament where at least two-thirds of the members owed their election to the landowners of fewer than a hundred boroughs. Wellesley continued to serve at Dublin Castle, voting with the government in the Irish parliament over the next two years. He became a captain on 30 January 1791, and was transferred to the 58th Regiment of Foot.

On 31 October, he transferred to the 18th Light Dragoons and it was during this period that he grew increasingly attracted to Kitty Pakenham, the daughter of Edward Pakenham, 2nd Baron Longford. She was described as being full of 'gaiety and charm'. In 1793, he sought her hand, but was turned down by her brother Thomas, Earl of Longford, who considered Wellesley to be a young man, in debt, with very poor prospects. An aspiring amateur musician, Wellesley, devastated by the rejection, burnt his violins in anger, and resolved to pursue a military career in earnest. He became a major by purchase in the 33rd Regiment in 1793. A few months later, in September, his brother lent him more money and with it he purchased a lieutenant-colonelcy in the 33rd.

In 1793, the Duke of York was sent to Flanders in command of the British contingent of an allied force destined for the invasion of France. In June 1794, Wellesley with the 33rd regiment set sail from Cork bound for Ostend as part of an expedition bringing reinforcements for the army in Flanders. They arrived too late and joined the Duke of York as he was pulling back towards the Netherlands. On 15 September 1794, at the Battle of Boxtel, east of Breda, Wellington, in temporary command of his brigade, had his first experience of battle. During General Abercromby’s withdrawal in the face of superior French forces, the 33rd held off enemy cavalry, allowing neighbouring units to retreat safely. During the extremely harsh winter that followed, Wellesley and his regiment formed part of an allied force holding the defence line along the Waal River. The 33rd, along with the rest of the army, suffered heavy losses from sickness and exposure. Wellesley’s health was also affected by the damp environment. Though the campaign was to end disastrously, with the British army driven out of the United Provinces into Germany, Wellesley was to learn several valuable lessons, including the use of steady lines of infantry against advancing columns and of the merits of supporting sea-power. He understood that the failure of the campaign was due in part to the faults of the Leaders and the poor organisation at headquarters. He remarked later of his time in the Netherlands that "At least I learned what not to do, and that is always a valuable lesson".

Returning to England in March 1795, he was returned as a Member of Parliament for Trim for a second time. He hoped to be given the position of secretary of war in the new Irish government but the new lord-lieutenant, Lord Camden, was only able to offer him the post of Surveyor-General of the Ordnance. Declining the post, he returned to his regiment, now at Southampton preparing to set sail for the West Indies. After seven weeks at sea, a storm forced the fleet back to Poole. The 33rd was given time to recuperate and a few months later, Whitehall decided to send the regiment to India. Wellesley was promoted full colonel by seniority on 3 May 1796 and a few weeks later set sail for Calcutta with his regiment.

Arriving in Calcutta in February 1797 he spent several months there, before being sent on a brief expedition to the Philippines, where he established a list of new hygiene precautions for his men to deal with the unfamiliar climate. Returning in November to India, he learnt that his elder brother Richard, now known as Lord Mornington, had been appointed as the new Governor-General of India.

As part of the campaign to extend the rule of the British East India Company, the Fourth Anglo-Mysore War broke out in 1798 against the Sultan of Mysore, Tipu Sultan. Arthur's brother Richard ordered that an armed force be sent to capture Seringapatam and defeat Tipu. Under the command of General Harris, some 24,000 troops were dispatched to Madras (to join an equal force being sent from Bombay in the west). Arthur and the 33rd sailed to join them in August.

Immediately after their arrival at Seringapatam on 5 April 1799, the Battle of Seringapatam began and Wellesley was ordered to lead a night attack on the village of Sultanpettah, adjacent to the fortress to clear the way for the artillery. Because of the enemy's strong defensive preparations, and the darkness, with the resulting confusion, the attack failed with 25 casualties. Wellesley suffered a minor injury to his knee from a spent musket-ball. Although they would re-attack successfully the next day, after time to scout ahead the enemy's positions, the affair affected Wellesley. He resolved "never to attack an enemy who is preparing and strongly posted, and whose posts have not been reconnoitered by daylight".

Given independent command of a combined East India Company and British Army force, Wellesley ventured north to confront Waugh in June 1800, with an army of 8,000 infantry and cavalry, having learned that Waugh's forces numbered over 50,000, although the majority (around 30,000) were irregular light cavalry and unlikely to pose a serious threat to British infantry and artillery.

He was promoted to brigadier-general on 17 July 1801. He took residence within the Sultan's summer palace and reformed the tax and justice systems in his province to maintain order and prevent bribery. He also hunted down the mercenary and self-proclaimed 'King' Dhoondiah Waugh, who had escaped from prison in Seringapatam during the battle. Wellesley, with command of four regiments, defeated Dhoondiah's larger rebel force, along with Dhoondiah himself who was killed in the battle. He paid for the Future upkeep of Dhoondiah's orphaned son.

He received good news when in September 1802 he learnt that he had been promoted to the rank of major-general. Wellesley had been gazetted on 29 April 1802, but the news took several months to reach him by sea. He remained at Mysore until November when he was sent to command an army in the Second Anglo-Maratha War.

When he determined that a long defensive war would ruin his army, Wellesley decided to act boldly to defeat the numerically larger force of the Maratha Empire. With the logistic assembly of his army complete (24,000 men in total) he gave the order to break camp and attack the nearest Maratha fort on 8 August 1803. The fort surrendered on 12 August after an infantry attack had exploited an artillery-made breach in the wall. With the fort now in British control Wellesley was able to extend control southwards to the river Godavari.

Wellesley had grown tired of his time in India, remarking "I have served as long in India as any man ought who can serve anywhere else". In June 1804 he applied for permission to return home and as a reward for his Service in India he was made a Knight of the Bath in September. While in India, Wellesley had amassed a fortune of £42,000 (considerable at the time), consisting mainly of prize money from his campaign. When his brother's term as Governor-General of India ended in March 1805, the brothers returned together to England on HMS Howe. Arthur, coincidentally, stopped on his voyage at the little island of Saint Helena and stayed in the same building to which Napoleon I would later be exiled.

Wellesley then served in the abortive Anglo-Russian expedition to north Germany in 1805, taking a brigade to Elbe.

Upon his return from the Hanover Expedition in 1805, Wellesley received good news; owing to his new title and status, Kitty Pakenham's family had consented to his marrying her. Wellesley and Kitty were married in Dublin on 10 April 1806. The marriage would prove to be unsatisfactory and the two would spend years apart while Wellesley was campaigning. Kitty grew depressed, while Wellesley pursued other sexual and romantic partners. However the marriage produced two sons, Arthur, in 1807, and Charles, in 1808. They lived apart for most of the time and occupied separate rooms in the house when they were together. Her brother, Edward "Ned" Pakenham, served under Wellesley throughout the Peninsular War and Wellesley's regard for him helped to smooth his relations with Kitty, until Ned Pakenham's death at the Battle of New Orleans in 1815.

Wellesley was in Ireland in May 1807 when he heard of the British expedition to Denmark. He decided to go, stepping down from his political appointments and was appointed to command an infantry brigade in the Second Battle of Copenhagen which took place in August. He fought at the Køge, during which the men under his command took 1,500 prisoners, with Wellesley later present during the surrender.

Wellesley defeated the French at the Battle of Roliça and the Battle of Vimeiro in 1808 but was superseded in command immediately after the latter battle. General Dalrymple then signed the controversial Convention of Sintra, which stipulated that the Royal Navy transport the French army out of Lisbon with all their loot, and insisted on the association of the only available government minister, Wellesley. Dalrymple and Wellesley were recalled to Britain to face a Court of Enquiry. Wellesley had agreed to sign the preliminary armistice, but had not signed the convention, and was cleared.

Wellesley arrived in Lisbon on 22 April 1809 onboard HMS Surveillante, after narrowly escaping shipwreck. Reinforced, he took to the offensive. In the Second Battle of Porto he crossed the Douro river in a daylight coup de main, and routed Marshal Soult's French troops in Porto.

In 1810, a newly enlarged French army under Marshal André Masséna invaded Portugal. British opinion both at home and in the army was negative and there were suggestions that they must evacuate Portugal. Instead, Lord Wellington first slowed the French down at Buçaco; he then prevented them from taking the Lisbon Peninsula by the construction of his massive earthworks, the Lines of Torres Vedras, which had been assembled in complete secrecy and had flanks guarded by the Royal Navy. The baffled and starving French invasion forces retreated after six months. Wellington's pursuit was frustrated by a series of reverses inflicted by Marshal Ney in a much-lauded rear guard campaign.

His term was marked by Catholic emancipation: the granting of almost full civil rights to Catholics in Great Britain and Ireland. The change was prompted by the landslide by-election win of Daniel O'Connell, an Irish Catholic proponent of emancipation, who was elected despite not being legally allowed to sit in Parliament. In the House of Lords, facing stiff opposition, Wellington spoke for Catholic Emancipation, and according to some sources, gave one of the best speeches of his career. He was born in Ireland and so had some understanding of the grievances of the Catholic communities there; as Chief Secretary, he had given an undertaking that the remaining Penal Laws would only be enforced as "mildly" as possible. In 1811 Catholic Soldiers were given freedom of worship and 18 years later the Catholic Relief Act 1829 was passed with a majority of 105. Many Tories voted against the Act, and it passed only with the help of the Whigs. Wellington had threatened to resign as Prime Minister if the King (George IV) did not give his Royal Assent.

In 1812, Wellington finally captured Ciudad Rodrigo by a rapid movement as the French went into winter quarters, storming it before they could react. He then moved south quickly, besieged the fortress of Badajoz for a month and captured it during one bloody night. On viewing the aftermath of the Storming of Badajoz, Wellington lost his composure and cried at the sight of the bloody Carnage in the breaches.

In 1813, Wellington led a new offensive, this time against the French line of communications. He struck through the hills north of Burgos, the Tras os Montes, and switched his supply line from Portugal to Santander on Spain's north coast; this led to the French abandoning Madrid and Burgos. Continuing to outflank the French lines, Wellington caught up with and smashed the army of King Joseph Bonaparte in the Battle of Vitoria, for which he was promoted to field marshal on 21 June. He personally led a column against the French centre, while other columns commanded by Sir Thomas Graham, Rowland Hill and the Earl of Dalhousie looped around the French right and left (this battle became the subject of Beethoven's opus 91, Wellington's Victory). The British troops broke ranks to loot the abandoned French wagons instead of pursuing the beaten foe. This gross abandonment of discipline caused an enraged Wellington to write in a famous dispatch to Earl Bathurst, "We have in the Service the scum of the earth as Common soldiers".

In late 1814, the Prime Minister wanted him to take command in Canada and with the assignment of winning the War of 1812 against the United States. Wellesley replied that he would go to America, but he believed that he was needed more in Europe. He stated:

Wellington always rose early; he "couldn't bear to lie awake in bed", even if the army was not on the march. Even when he returned to civilian life after 1815, he slept in a camp bed, reflecting his lack of regard for creature comforts; it remains on display in Walmer Castle. General Miguel de Álava complained that Wellington said so often that the army would march "at daybreak" and dine on "cold meat", that he began to dread those two phrases. While on campaign, he seldom ate anything between breakfast and dinner. During the retreat to Portugal in 1811, he subsisted on "cold meat and bread", to the despair of his staff who dined with him. He was, however, renowned for the quality of the wine which he drank and served, often drinking a bottle with his dinner (not a great quantity by the standards of his day).

The Waterloo Medal was duly authorised and distributed to all ranks in 1816.

Wellington entered politics again when he was appointed Master-General of the Ordnance in the Tory government of Lord Liverpool on 26 December 1818. He also became Governor of Plymouth on 9 October 1819. He was appointed Commander-in-Chief of the British Army on 22 January 1827 and Constable of the Tower of London on 5 February 1827.

Following an incident when, as Master-General of the Ordnance he had been close to a large explosion, Wellington begun to experience deafness and other ear-related problems. In 1822, he had an operation to improve the hearing of the left ear. The result, however, was that he became permanently deaf on that side. It is claimed that he was "never quite well afterwards".

Wellington had a "vigorous sexual appetite" and many amorous liaisons during his marriage to Kitty. He enjoyed the company of intellectual and attractive women for many decades, particularly after the Battle of Waterloo and his subsequent ambassadorial position in Paris. The British press lampooned this side of the national hero. In 1824, one liaison came back to haunt him, when Wellington received a letter from a publisher offering to refrain from issuing an edition of the rather racy memoirs of one of his mistresses Harriette Wilson, in exchange for financial consideration. It is said that the Duke promptly returned the letter, after scrawling across it, "Publish and be damned". However, Hibbert notes in his biography that the letter can be found among the Duke's papers, with nothing written on it. It is certain that Wellington did reply, and the tone of a further letter from the publisher, quoted by Longford, suggests that he had refused in the strongest language to submit to blackmail.

Wellington served as the leader of the Conservative party in the House of Lords, 1828–1846. Some historians have belittled him as a befuddled reactionary, but a consensus in the late 20th century depicts him as a shrewd operator who hid his cleverness behind the facade of a poorly informed old soldier. Wellington worked to transform the Lords from unstinting support of the Crown to an active player in political maneuvering, with a commitment to the landed aristocracy. He used his London residence as a venue for intimate dinners and private consultations, together with extensive correspondence that kept him in close touch with party Leaders in the Commons, and the main persona in the Lords. He gave public rhetorical support to Ultra-Tory anti-reform positions, but then deftly changed positions toward the party's center, especially when Peel needed support from the upper house. Wellington's success was based on the 44 Elected peers from Scotland and Ireland, whose election he controlled.

The Earl of Winchilsea accused the Duke of "an insidious design for the infringement of our liberties and the introduction of Popery into every department of the State". Wellington responded by immediately challenging Winchilsea to a duel. On 21 March 1829, Wellington and Winchilsea met on Battersea fields. When the time came to fire, the Duke took aim and Winchilsea kept his arm down. The Duke fired wide to the right. Accounts differ as to whether he missed on purpose, an act known in dueling as a delope. Wellington claimed he did. However, he was noted for his poor aim and reports more sympathetic to Winchilsea claimed he had aimed to kill. Winchilsea did not fire, a plan he and his second had almost certainly decided upon before the duel. Honour was saved and Winchilsea wrote Wellington an apology.

Wellington's government fell in 1830. In the summer and autumn of that year, a wave of riots swept the country. The Whigs had been out of power for most years since the 1770s, and saw political reform in response to the unrest as the key to their return. Wellington stuck to the Tory policy of no reform and no expansion of suffrage, and as a result lost a vote of no confidence on 15 November 1830.

Kitty died of cancer in 1831; despite their generally unhappy relations, which had led to an effective separation, Wellington was said to have been greatly saddened by her death, his one comfort being that after "half a lifetime together, they had come to understand each other at the end". He had found consolation for his unhappy marriage in his warm friendship with the diarist Harriet Arbuthnot, wife of his colleague Charles Arbuthnot. Harriet's death in the cholera epidemic of 1834 was almost as great a blow to Wellington as it was to her husband. The two widowers spent their last years together at Apsley House.

This commonly used nickname originally related to his consistent political resolve rather than to any particular incident. In various cases its editorial use appears to be disparaging. It is likely that its use became more widespread after an incident in 1832 in which he installed metal shutters to prevent rioters breaking windows at Apsley House. The term may have been made increasingly popular by Punch cartoons published in 1844–45.

During debate on the Jewish Civil Disabilities Repeal Bill, Wellington, who opposed the Bill, stated in Parliament on 1 August 1833: "... this is a Christian country and a Christian legislature, and that the effect of this measure would be to remove that peculiar character." And "I see no ground whatever for passing the Bill; and shall, therefore, vote against it." The Bill was defeated, 104 votes against, and 54 for.

Wellington was gradually superseded as leader of the Tories by Robert Peel, while the party evolved into the Conservatives. When the Tories were returned to power in 1834, Wellington declined to become Prime Minister because he thought membership in Commons had become essential. The king reluctantly approved Peel, who was in Italy. So for three weeks in November and December 1834, Wellington acted as interim leader, taking the responsibilities of Prime Minister and most of the other ministries. In Peel's first cabinet (1834–1835), Wellington became Foreign Secretary, while in the second (1841–1846) he was a Minister without Portfolio and Leader of the House of Lords. Wellington was also re-appointed Commander-in-Chief of the British Army on 15 August 1842 following the resignation of Lord Hill.

The Conservative Party had split over the Repeal of the Corn Laws in 1846, with Wellington and most of the former Cabinet still supporting Peel, but most of the MPs led by Lord Derby supporting a protectionist stance. Early in 1852 Wellington, by then very deaf, gave Derby's first government its nickname by shouting "Who? Who?" as the list of inexperienced Cabinet Ministers was read out in the House of Lords.

He became Chief Ranger and Keeper of Hyde Park and St. James's Park on 31 August 1850. He was also colonel of the 33rd Regiment of Foot from 1 February 1806 and colonel of the Grenadier Guards from 22 January 1827.

He was also a remarkably practical man who spoke concisely. In 1851, it was discovered that there were a great many sparrows flying about in the Crystal Palace just before the Great Exhibition was to open. His advice to Queen Victoria was "Sparrowhawks, ma'am".

Although in life he hated travelling by rail (after witnessing the death of william Huskisson, one of the first railway accident casualties), his body was then taken by train to London, where he was given a state funeral – one of only a handful of British subjects to be honoured in that way (other examples are Lord Nelson and Sir Winston Churchill) – and the last heraldic state funeral to be held in Britain. The funeral took place on 18 November 1852. At his funeral there was hardly any space to stand because of the number of people attending, and the effusive praise given him in Tennyson's "Ode on the Death of the Duke of Wellington" attests to his stature at the time of his death. He was buried in a sarcophagus of luxulyanite in St Paul's Cathedral next to Lord Nelson. A bronze memorial was sculpted by Alfred Stevens, and features two intricate supports: "Truth tearing the tongue out of the mouth of False-hood", and "Valour trampling Cowardice underfoot". Stevens did not live to see it placed in its home under one of the great arches of the Cathedral.

Owing to its links with Wellington, as the former commanding officer and colonel of the regiment, the title "33rd (The Duke of Wellington's) Regiment" was granted to the 33rd Regiment of Foot, on 18 June 1853 (the 38th anniversary of the Battle of Waterloo) by Queen Victoria. Wellington's battle record is exemplary; he participated in some 60 battles during the course of his military career.

After extensive and careful logistic preparation (which would become one of Wellesley's main attributes) the 33rd left with the main force in December and travelled across 250 miles (402 km) of jungle from Madras to Mysore. On account of his brother, during the journey, Wellesley was given an additional command, that of chief advisor to the Nizam of Hyderabad's army (sent to accompany the British force). This position was to cause friction among many of the senior officers (some of whom were senior to Wellesley). Much of this friction was put to rest after the Battle of Mallavelly, some 20 miles (32 km) from Seringapatam, in which Harris's army attacked a large part of the sultan's army. During the battle, Wellesley led his men, in a line of battle of two ranks, against the enemy to a gentle ridge and gave the order to fire. After an extensive repetition of volleys, followed by a bayonet charge, the 33rd, in conjunction with the rest of Harris's force, forced Tipu's infantry to retreat.

Further to the west, 1,500 British Foot Guards under Maitland were lying down to protect themselves from the French artillery. As two battalions of Chasseurs approached, the second prong of the Imperial Guard's attack, Maitland's guardsmen rose and devastated them with point-blank volleys. The Chasseurs deployed to counter-attack, but began to waver. A bayonet charge by the Foot Guards then broke them. The third prong, a fresh Chasseur battalion, now came up in support. The British guardsmen retreated with these Chasseurs in pursuit, but the latter were halted as the 52nd Light Infantry wheeled in line onto their flank and poured a devastating fire into them and then charged. Under this onslaught they too broke.

After his death, Irish and English newspapers disputed whether Wellington had been born an Irishman or an Englishman. In 2002, he was number 15 in the BBC's poll of the 100 Greatest Britons.

At about 16:30, the first Prussian corps arrived. Commanded by Freiherr von Bülow, IV Corps arrived as the French cavalry attack was in full spate. Bülow sent the 15th Brigade to link up with Wellington's left flank in the Frichermont–La Haie area while the brigade's horse artillery battery and additional brigade artillery deployed to its left in support. Napoleon sent Lobau's corps to intercept the rest of Bülow's IV Corps proceeding to Plancenoit. The 15th Brigade sent Lobau's corps into retreat to the Plancenoit area. Von Hiller's 16th Brigade also pushed forward with six battalions against Plancenoit. Napoleon had dispatched all eight battalions of the Young Guard to reinforce Lobau, who was now seriously pressed by the enemy. Napoleon's Young Guard counter-attacked and, after very hard fighting, secured Plancenoit, but were themselves counter-attacked and driven out. Napoleon then resorted to sending two battalions of the Middle and Old Guard into Plancenoit and after ferocious fighting they recaptured the village. The French cavalry attacked the British infantry squares many times, each at heavy cost to the French but with few British casualties. Ney himself was displaced from his horse four times. Eventually it became obvious, even to Ney, that cavalry alone were achieving little. Belatedly, he organised a combined-arms attack, using Bachelu's division and Tissot's regiment of Foy's division from Reille's II Corps plus those French cavalry that remained in a fit state to fight. This assault was directed along much the same route as the previous heavy cavalry attacks.

The campaign led to numerous other controversies, especially concerning the Prussians. For example: Were Wellington's troop dispositions prior to Napoleon's invasion of Belgium sound? Did Wellington somehow mislead or betray Blücher by promising, then failing, to come directly to Blücher's aid at Ligny? Who deserved the lion's share of credit for the victory—Wellington or the Prussians? These and other such issues concerning Blücher's, Wellington's, and Napoleon's decisions during the campaign were the subject of a major strategic-level study by the famous Prussian political-military theorist Carl von Clausewitz, Feldzug von 1815: Strategische Uebersicht des Feldzugs von 1815, English title: The Campaign of 1815: Strategic Overview of the Campaign. Written c.1827, this study was Clausewitz's last such work and is widely considered to be the best Example of Clausewitz's mature theories concerning such analyses. It attracted the attention of Wellington's staff, who prompted the Duke to write his only published essay on the campaign (other than his immediate, official after-action report, "The Waterloo Dispatch"), his 1842 "Memorandum on the Battle of Waterloo". While Wellington disputed Clausewitz on several points, the Prussian Writer largely absolved Wellington of accusations levelled against him by nationalistic German axe-grinders. This exchange with Clausewitz was quite famous in Britain in the 19th century (it was heavily discussed in, for Example, Chesney's Waterloo Lectures (1868).) It seems, however, to have been systematically ignored by British historians writing since 1914, which is odd considering that it was one of only two discussions of the battle that Wellington wrote. The explanation, unfortunately, is probably that it drew too much attention to the decisive German role in Wellington's victory—which Wellington himself was perfectly happy to acknowledge, but which became an awkward subject given Anglo-German hostilities in the 20th century.

Meanwhile, at approximately the same time as Ney's combined-arms assault on the centre-right of Wellington's line, Napoleon ordered Ney to capture La Haye Sainte at whatever the cost. Ney accomplished this with what was left of D'Erlon's corps soon after 18:00. Ney then moved horse artillery up towards Wellington's centre and began to destroy the infantry squares at short-range with canister. This all but destroyed the 27th (Inniskilling) Regiment, and the 30th and 73rd Regiments suffered such heavy losses that they had to combine to form a viable square. Wellington's centre was now on the verge of collapse and wide open to an attack from the French. Luckily for Wellington, Pirch I's and Zieten's corps of the Prussian Army were now at hand. Zieten's corps permitted the two fresh cavalry brigades of Vivian and Vandeleur on Wellington's extreme left to be moved and posted behind the depleted centre. Pirch I Corps then proceeded to support Bülow and together they regained possession of Plancenoit, and once more the Charleroi road was swept by Prussian round shot. The value of this reinforcement at this particular moment can hardly be overestimated.

The French invaded Belgium, with Napoleon defeating the Prussians at Ligny, and Marshal Ney engaging indecisively with Wellington, at the Battle of Quatre Bras. The Prussians retreated 18 miles north to Wavre whilst Wellington’s Anglo-Allied army withdrew 15 miles north to a site he had noted the previous year as favourable for a battle: the north ridge of a shallow valley on the Brussels road, just south of the small town of Waterloo. On 17 June there was torrential rain, which severely hampered movement and had a considerable effect the next day, 18 June, when the Battle of Waterloo was fought. This was the first time Wellington had encountered Napoleon; he commanded an Anglo-Dutch-German army that consisted of approximately 73,000 troops, 26,000 of whom were British, approximately 30 percent of that 26,000 were Irish.