Oppenheimer was born in New York City on April 22, 1904, to Julius Oppenheimer, a wealthy Jewish textile importer who had immigrated to the United States from Germany in 1888, and Ella Friedman, a Painter. Julius came to America with no money, no baccalaureate studies, and no knowledge of the English language. He got a job in a textile company and within a decade was an executive with the company. Ella was from Baltimore. The Oppenheimers were non-observant Ashkenazi Jews. In 1912 the family moved to an apartment on the 11th floor of 155 Riverside Drive, near West 88th Street, Manhattan, an area known for luxurious mansions and townhouses. Their art collection included works by Pablo Picasso and Édouard Vuillard, and at least three original paintings by Vincent van Gogh. Robert had a younger brother, Frank, who also became a Physicist.

Oppenheimer was initially educated at Alcuin Preparatory School, and in 1911 he entered the Ethical Culture Society School. This had been founded by Felix Adler to promote a form of ethical training based on the Ethical Culture movement, whose motto was "Deed before Creed". His father had been a member of the Society for many years, serving on its board of trustees from 1907 to 1915. Oppenheimer was a versatile scholar, interested in English and French literature, and particularly in mineralogy. He completed the third and fourth grades in one year, and skipped half the eighth grade. During his final year, he became interested in chemistry. He entered Harvard College one year after graduation, at age 18, because he suffered an attack of colitis while prospecting in Joachimstal during a family summer vacation in Europe. To help him recover from the illness, his father enlisted the help of his English Teacher Herbert Smith who took him to New Mexico, where Oppenheimer fell in love with horseback riding and the southwestern United States.

During the 1920s, Oppenheimer remained uninformed on worldly matters. He claimed that he did not read newspapers or Listen to the radio, and had only learned of the Wall Street crash of 1929 some six months after it occurred while on a walk with Ernest Lawrence. He once remarked that he never cast a vote until the 1936 election. However, from 1934 on, he became increasingly concerned about politics and international affairs. In 1934, he earmarked three percent of his salary—about $100 a year—for two years to support German physicists fleeing from Nazi Germany. During the 1934 West Coast Waterfront Strike, he and some of his students, including Melba Phillips and Bob Serber, attended a longshoremen's rally. Oppenheimer repeatedly attempted to get Serber a position at Berkeley but was blocked by Birge, who felt that "one Jew in the department was enough".

In 1924 Oppenheimer was informed that he had been accepted into Christ's College, Cambridge. He wrote to Ernest Rutherford requesting permission to work at the Cavendish Laboratory. Bridgman provided Oppenheimer with a recommendation, which conceded that Oppenheimer's clumsiness in the laboratory made it apparent his forte was not experimental but rather theoretical physics. Rutherford was unimpressed, but Oppenheimer went to Cambridge in the hope of landing another offer. He was ultimately accepted by J. J. Thomson on condition that he complete a basic laboratory course. He developed an antagonistic relationship with his tutor, Patrick Blackett, who was only a few years his senior. While on vacation, as recalled by his friend Francis Fergusson, Oppenheimer once confessed that he had left an apple doused with Noxious chemicals on Blackett's desk. While Ferguson's account is the only detailed version of this event, Oppenheimer's parents were alerted by the university authorities who considered placing him on probation, a fate prevented by his parents successfully lobbying the authorities.

Initially, his major interest was the theory of the continuous spectrum and his first published paper, in 1926, concerned the quantum theory of molecular band spectra. He developed a method to carry out calculations of its transition probabilities. He calculated the photoelectric effect for hydrogen and X-rays, obtaining the absorption coefficient at the K-edge. His calculations accorded with observations of the X-ray absorption of the sun, but not helium. Years later it was realized that the sun was largely composed of hydrogen and that his calculations were indeed correct.

Oppenheimer was awarded a United States National Research Council fellowship to the California Institute of Technology (Caltech) in September 1927. Bridgman also wanted him at Harvard, so a compromise was reached whereby he split his fellowship for the 1927–28 academic year between Harvard in 1927 and Caltech in 1928. At Caltech he struck up a close friendship with Linus Pauling, and they planned to mount a joint attack on the nature of the chemical bond, a field in which Pauling was a pioneer, with Oppenheimer supplying the mathematics and Pauling interpreting the results. Both the collaboration and their friendship were nipped in the bud when Pauling began to suspect Oppenheimer of becoming too close to his wife, Ava Helen Pauling. Once, when Pauling was at work, Oppenheimer had arrived at their home and invited Ava Helen to join him on a tryst in Mexico. Though she refused and reported the incident to her husband, the invitation, and her apparent nonchalance about it, disquieted Pauling and he ended his relationship with Oppenheimer. Oppenheimer later invited him to become head of the Chemistry Division of the Manhattan Project, but Pauling refused, saying he was a pacifist.

In the autumn of 1928, Oppenheimer visited Paul Ehrenfest's institute at the University of Leiden, the Netherlands, where he impressed by giving lectures in Dutch, despite having little experience with the language. There he was given the nickname of Opje, later anglicized by his students as "Oppie". From Leiden he continued on to the Swiss Federal Institute of Technology (ETH) in Zurich to work with Wolfgang Pauli on quantum mechanics and the continuous spectrum. Oppenheimer respected and liked Pauli and may have emulated his personal style as well as his critical approach to problems.

In the late 1930s Oppenheimer became interested in astrophysics, probably through his friendship with Richard Tolman, resulting in a series of papers. In the first of these, a 1938 paper co-written with Robert Serber entitled "On the Stability of Stellar Neutron Cores", Oppenheimer explored the properties of white dwarfs. This was followed by a paper co-written with one of his students, George Volkoff, "On Massive Neutron Cores", in which they demonstrated that there was a limit, the so-called Tolman–Oppenheimer–Volkoff limit, to the mass of stars beyond which they would not remain stable as neutron stars and would undergo gravitational collapse. Finally, in 1939, Oppenheimer and another of his students, Hartland Snyder, produced a paper "On Continued Gravitational Attraction", which predicted the existence of what are today known as black holes. After the Born–Oppenheimer approximation paper, these papers remain his most cited, and were key factors in the rejuvenation of astrophysical research in the United States in the 1950s, mainly by John A. Wheeler.

Oppenheimer's mother died in 1931, and he became closer to his father who, although still living in New York, became a frequent visitor in California. When his father died in 1937 leaving $392,602 to be divided between Oppenheimer and his brother Frank, Oppenheimer immediately wrote out a will leaving his estate to the University of California for graduate scholarships. Like many young intellectuals in the 1930s, he supported social reforms that were later alleged to be communist ideas. He donated to many progressive efforts which were later branded as "left-wing" during the McCarthy era. The majority of his allegedly radical work consisted of hosting fundraisers for the Republican cause in the Spanish Civil War and other anti-fascist activity. He never openly joined the Communist Party, though he did pass money to liberal causes by way of acquaintances who were alleged to be Party members. In 1936, Oppenheimer became involved with Jean Tatlock, the daughter of a Berkeley literature professor and a student at Stanford University School of Medicine. The two had similar political views; she wrote for the Western Worker, a Communist Party newspaper.

Oppenheimer's diverse interests sometimes interrupted his focus on science. In 1933 he learned Sanskrit and met the Indologist Arthur W. Ryder at Berkeley. He read the Bhagavad Gita in the original Sanskrit, and later he cited it as one of the books that most shaped his philosophy of life. His close confidant and colleague, Nobel Prize winner Isidor Rabi, later gave his own interpretation:

Oppenheimer worked with his first doctoral student, Melba Phillips, on calculations of artificial radioactivity under bombardment by deuterons. When Ernest Lawrence and Edwin McMillan bombarded nuclei with deuterons they found the results agreed closely with the predictions of George Gamow, but when higher energies and heavier nuclei were involved, the results did not conform to the theory. In 1935, Oppenheimer and Phillips worked out a theory now known as the Oppenheimer–Phillips process to explain the results, a theory still in use today.

He worked closely with Nobel Prize-winning experimental Physicist Ernest O. Lawrence and his cyclotron pioneers, helping them understand the data their machines were producing at the Lawrence Berkeley National Laboratory. In 1936 Berkeley promoted him to full professor at a salary of $3300 per annum. In return he was asked to curtail his teaching at Caltech, so a compromise was reached whereby Berkeley released him for six weeks each year, enough to teach one term at Caltech.

The FBI under J. Edgar Hoover had been following Oppenheimer since before the war, when he showed Communist sympathies as a professor at Berkeley and had been close to members of the Communist Party, including his wife and brother. He had been under close surveillance since the early 1940s, his home and office bugged, his phone tapped and his mail opened. The FBI furnished Oppenheimer's political enemies with incriminating evidence about his Communist ties. These enemies included Strauss, an AEC commissioner who had long harbored resentment against Oppenheimer both for his activity in opposing the hydrogen bomb and for his humiliation of Strauss before Congress some years earlier; regarding Strauss's opposition to the export of radioactive isotopes to other nations, Oppenheimer had memorably categorized these as "less important than electronic devices but more important than, let us say, vitamins".

On October 9, 1941, shortly before the United States entered World War II, President Franklin D. Roosevelt approved a crash program to develop an atomic bomb. In May 1942, National Defense Research Committee Chairman James B. Conant, who had been one of Oppenheimer's lecturers at Harvard, invited Oppenheimer to take over work on fast neutron calculations, a task that Oppenheimer threw himself into with full vigor. He was given the title "Coordinator of Rapid Rupture", specifically referring to the propagation of a fast neutron chain reaction in an atomic bomb. One of his first acts was to host a summer school for bomb theory at his building in Berkeley. The mix of European physicists and his own students—a group including Robert Serber, Emil Konopinski, Felix Bloch, Hans Bethe and Edward Teller—busied themselves calculating what needed to be done, and in what order, to make the bomb.

Oppenheimer and Groves decided that for security and cohesion they needed a centralized, secret research laboratory in a remote location. Scouting for a site in late 1942, Oppenheimer was drawn to New Mexico, not far from his ranch. On November 16, 1942, Oppenheimer, Groves and others toured a prospective site. Oppenheimer feared that the high cliffs surrounding the site would make his people feel claustrophobic, while the Engineers were concerned with the possibility of flooding. He then suggested and championed a site that he knew well: a flat mesa near Santa Fe, New Mexico, which was the site of a private boys' school called the Los Alamos Ranch School. The Engineers were concerned about the poor access road and the water supply, but otherwise felt that it was ideal. The Los Alamos Laboratory was built on the site of the school, taking over some of its buildings, while many others were erected in great haste. There Oppenheimer assembled a group of the top physicists of the time, which he referred to as the "luminaries".

This led to outrage by the scientific community and Teller's virtual expulsion from academic science. Ernest Lawrence refused to testify on the grounds that he was suffering from an attack of ulcerative colitis, but an interview transcript in which he condemned Oppenheimer was presented as evidence in his absence. Groves, threatened by the FBI as having been potentially part of a coverup about the Chevalier contact in 1943, likewise testified against Oppenheimer. Many top Scientists, as well as government and military figures, testified on Oppenheimer's behalf. Inconsistencies in his testimony and his erratic behavior on the stand, at one point saying he had given a "cock and bull story" and that this was because he "was an idiot", convinced some that he was unstable, unreliable and a possible security risk. Oppenheimer's clearance was revoked one day before it was due to lapse anyway. Isidor Rabi's comment was that Oppenheimer was merely a government consultant at the time anyway and that if the government "didn't want to consult the guy, then don't consult him".

Throughout the development of the atomic bomb, Oppenheimer was under investigation by both the FBI and the Manhattan Project's internal security arm for his past left-wing associations. He was followed by Army security agents during a trip to California in June 1943 to visit his former girlfriend, Jean Tatlock, who was suffering from depression. Oppenheimer spent the night in her apartment. Tatlock committed suicide on January 4, 1944, which left Oppenheimer deeply grieved. In August 1943, he volunteered to Manhattan Project security agents that George Eltenton, whom he did not know, had solicited three men at Los Alamos for nuclear secrets on behalf of the Soviet Union. When pressed on the issue in later interviews, Oppenheimer admitted that the only person who had approached him was his friend Haakon Chevalier, a Berkeley professor of French literature, who had mentioned the matter privately at a dinner at Oppenheimer's house. Brigadier General Leslie R. Groves, Jr., the Director of the Manhattan Project, thought Oppenheimer was too important to the project to be ousted over this suspicious behavior. On July 20, 1943, he wrote to the Manhattan Engineer District:

In November 1945, Oppenheimer left Los Alamos to return to Caltech, but he soon found that his heart was no longer in teaching. In 1947, he accepted an offer from Lewis Strauss to take up the directorship of the Institute for Advanced Study in Princeton, New Jersey. This meant moving back east and leaving Ruth Tolman, the wife of his friend Richard Tolman, with whom he had begun an affair after leaving Los Alamos. The job came with a salary of $20,000 per annum, plus rent-free accommodation in the director's house, a 17th-century manor with a cook and groundskeeper, surrounded by 265 acres (107 ha) of woodlands.

For his services as Director of Los Alamos, Oppenheimer was awarded the Medal for Merit from President Harry S Truman in 1946.

After the Atomic Energy Commission (AEC) came into being in 1947 as a civilian agency in control of nuclear research and weapons issues, Oppenheimer was appointed as the Chairman of its General Advisory Committee (GAC). From this position he advised on a number of nuclear-related issues, including project funding, laboratory construction and even international policy—though the GAC's advice was not always heeded. As Chairman of the GAC, Oppenheimer lobbied vigorously for international arms control and funding for basic science, and attempted to influence policy away from a heated arms race. When the government questioned whether to pursue a crash program to develop an atomic weapon based on nuclear fusion—the hydrogen bomb—Oppenheimer initially recommended against it, though he had been in favor of developing such a weapon during the Manhattan Project. He was motivated partly by ethical concerns, feeling that such a weapon could only be used strategically against civilian targets, resulting in millions of deaths. He was also motivated by practical concerns, however, as at the time there was no workable design for a hydrogen bomb. Oppenheimer felt that resources would be better spent creating a large force of fission weapons. He and others were especially concerned about nuclear reactors being diverted from plutonium to tritium production. They were overridden by Truman, who announced a crash program after the Soviet Union tested their first atomic bomb in 1949. Oppenheimer and other GAC opponents of the project, especially James Conant, felt personally shunned and considered retiring from the committee. They stayed on, though their views on the hydrogen bomb were well known.

Oppenheimer had found himself in the middle of more than one controversy and power struggle in the years from 1949 to 1953. Edward Teller, who had been so uninterested in work on the atomic bomb at Los Alamos during the war that Oppenheimer had given him time instead to work on his own project of the hydrogen bomb, had eventually left Los Alamos in 1951 to help found, in 1952, a second laboratory at what would become the Lawrence Livermore National Laboratory. There, he could be free of Los Alamos control to develop the hydrogen bomb. Long-range thermonuclear "strategic" weapons delivered by jet bombers would necessarily be under control of the new United States Air Force (USAF). Oppenheimer had for some years pushed for smaller "tactical" nuclear weapons which would be more useful in a limited theater against enemy troops and which would be under control of the Army. The two services fought for control of nuclear weapons, often allied with different political parties. The USAF, with Teller pushing its program, gained ascendance in the Republican-controlled administration following the election of Dwight D. Eisenhower as President in 1952.

Oppenheimer published only five scientific papers, one of which was in biophysics, after World War II, and none after 1950. Murray Gell-Mann, a later Nobelist who, as a visiting scientist, worked with him at the Institute for Advanced Study in 1951, offered this opinion:

In 1951, however, Edward Teller and Mathematician Stanislaw Ulam developed what became known as the Teller-Ulam design for a hydrogen bomb. This new design seemed technically feasible and Oppenheimer changed his opinion about developing the weapon. As he later recalled:

In his speeches and public writings, Oppenheimer continually stressed the difficulty of managing the power of knowledge in a world in which the freedom of science to exchange ideas was more and more hobbled by political concerns. Oppenheimer delivered the Reith Lectures on the BBC in 1953, which were subsequently published as Science and the Common Understanding. In 1955 Oppenheimer published The Open Mind, a collection of eight lectures that he had given since 1946 on the subject of nuclear weapons and popular culture. Oppenheimer rejected the idea of nuclear gunboat diplomacy. "The purposes of this country in the field of foreign policy", he wrote, "cannot in any real or enduring way be achieved by coercion". In 1957 the philosophy and psychology departments at Harvard invited Oppenheimer to deliver the william James Lectures. An influential group of Harvard alumni led by Edwin Ginn that included Archibald Roosevelt protested against the decision. Some 1,200 people packed into Sanders Theatre to hear Oppenheimer's six lectures, entitled "The Hope of Order". Oppenheimer delivered the Whidden Lectures at McMaster University in 1962, and these were published in 1964 as The Flying Trapeze: Three Crises for Physicists.

When Oppenheimer was ejected from his position of political influence in 1954, he symbolized for many the folly of Scientists thinking they could control how others would use their research. He has also been seen as symbolizing the dilemmas involving the moral responsibility of the scientist in the nuclear world. The hearings were motivated both by politics, as Oppenheimer was seen as a representative of the previous administration, and by personal considerations stemming from his enmity with Lewis Strauss. The ostensible reason for the hearing and the issue that aligned Oppenheimer with the liberal intellectuals, Oppenheimer's opposition to hydrogen bomb development, was based as much on technical grounds as on moral ones. Once the technical considerations were resolved, he supported Teller's hydrogen bomb because he believed that the Soviet Union would inevitably construct one too. Rather than consistently oppose the "Red-baiting" of the late 1940s and early 1950s, Oppenheimer testified against some of his former colleagues and students, both before and during his hearing. In one incident, his damning testimony against former student Bernard Peters was selectively leaked to the press. Historians have interpreted this as an attempt by Oppenheimer to please his colleagues in the government and perhaps to divert attention from his own previous left-wing ties and those of his brother. In the end it became a liability when it became clear that if Oppenheimer had really doubted Peters' loyalty, his recommending him for the Manhattan Project was reckless, or at least contradictory.

Popular depictions of Oppenheimer view his security struggles as a confrontation between right-wing militarists (symbolized by Teller) and left-wing intellectuals (symbolized by Oppenheimer) over the moral question of weapons of mass destruction. The question of the scientists' responsibility toward humanity inspired Bertolt Brecht's drama Galileo (1955), left its imprint on Friedrich Dürrenmatt's Die Physiker, and is the basis of the opera Doctor Atomic by John Adams (2005), which was commissioned to portray Oppenheimer as a modern-day Faust. Heinar Kipphardt's play In the Matter of J. Robert Oppenheimer, after appearing on West German television, had its theatrical release in Berlin and Munich in October 1964. Oppenheimer's objections resulted in an exchange of correspondence with Kipphardt, in which the Playwright offered to make corrections but defended the play. It premiered in New York in June 1968, with Joseph Wiseman in the Oppenheimer role. New York Times theater critic Clive Barnes called it an "angry play and a partisan play" that sided with Oppenheimer but portrayed the scientist as a "tragic fool and genius". Oppenheimer had difficulty with this portrayal. After reading a transcript of Kipphardt's play soon after it began to be performed, Oppenheimer threatened to sue the Playwright, decrying "improvisations which were contrary to history and to the nature of the people involved". Later Oppenheimer told an interviewer:

The rehabilitation implied by the award was partly symbolic, as Oppenheimer still lacked a security clearance and could have no effect on official policy, but the award came with a $50,000 tax-free stipend, and its award outraged many prominent Republicans in Congress. The late President Kennedy's widow Jacqueline, still living in the White House, made it a point to meet with Oppenheimer to tell him how much her husband had wanted him to have the medal. While still a senator in 1959, Kennedy had been instrumental in voting to narrowly deny Oppenheimer's enemy Lewis Strauss a coveted government position as Secretary of Commerce, effectively ending Strauss's political career. This was partly due to lobbying by the scientific community on behalf of Oppenheimer.

Oppenheimer was increasingly concerned about the potential danger that scientific inventions could pose to humanity. He joined with Albert Einstein, Bertrand Russell, Joseph Rotblat and other eminent Scientists and academics to establish what would eventually, in 1960, become the World Academy of Art and Science. Significantly, after his public humiliation, he did not sign the major open protests against nuclear weapons of the 1950s, including the Russell–Einstein Manifesto of 1955, nor, though invited, did he attend the first Pugwash Conferences on Science and World Affairs in 1957.

Deprived of political power, Oppenheimer continued to lecture, write and work on physics. He toured Europe and Japan, giving talks about the history of science, the role of science in society, and the nature of the universe. In September 1957, France made him an Officer of the Legion of Honor, and on May 3, 1962, he was elected a Foreign Member of the Royal Society in Britain. At the urging of many of Oppenheimer's political friends who had ascended to power, President John F. Kennedy awarded Oppenheimer the Enrico Fermi Award in 1963 as a gesture of political rehabilitation. Edward Teller, the winner of the previous year's award, had also recommended Oppenheimer receive it, in the hope that it would heal the rift between them. A little over a week after Kennedy's assassination, his successor, President Lyndon Johnson, presented Oppenheimer with the award, "for contributions to theoretical physics as a Teacher and originator of ideas, and for leadership of the Los Alamos Laboratory and the atomic Energy program during critical years". Oppenheimer told Johnson: "I think it is just possible, Mr. President, that it has taken some charity and some courage for you to make this award today."

In 1965, he was persuaded to quote again for a television broadcast:

Oppenheimer was a chain smoker who was diagnosed with throat cancer in late 1965. After inconclusive surgery, he underwent unsuccessful radiation treatment and chemotherapy late in 1966. He fell into a coma on February 15, 1967, and died at his home in Princeton, New Jersey, on February 18, aged 62. A memorial Service was held a week later at Alexander Hall on the campus of Princeton University. The Service was attended by 600 of his scientific, political and military associates that included Bethe, Groves, Kennan, Lilienthal, Rabi, Smyth and Wigner. His brother Frank and the rest of his family were also there, as was the Historian Arthur M. Schlesinger, Jr., the Novelist John O'Hara, and George Balanchine, the Director of the New York City Ballet. Bethe, Kennan and Smyth gave brief eulogies. Oppenheimer was cremated and his ashes were placed into an urn. His wife Kitty took his ashes to St. John and dropped the urn into the sea off the coast, within sight of the beach house.

In October 1972, Kitty died at age 62 from an intestinal infection that was complicated by a pulmonary embolism. Oppenheimer's ranch in New Mexico was then inherited by their son Peter, and the beach property was inherited by their daughter Katherine "Toni" Oppenheimer Silber. Toni was refused security clearance for her chosen vocation as a United Nations translator after the FBI brought up the old charges against her father. In January 1977 (three months after the end of her second marriage), she committed suicide at age 32; her ex-husband found her hanging from a beam in her family beach house. She left the property to "the people of St. John for a public park and recreation area". The original house was built too close to the coast and succumbed to a hurricane. Today the Virgin Islands Government maintains a Community Center in the area.

The 1980 BBC TV serial Oppenheimer, starring Sam Waterston, won three BAFTA Television Awards. The Day After Trinity, a 1980 documentary about J. Robert Oppenheimer and the building of the atomic bomb, was nominated for an Academy Award and received a Peabody Award. Oppenheimer's life has been explored in the play Oppenheimer by Tom Morton-Smith. In addition to his use by authors of fiction, there are numerous biographies, including American Prometheus: The Triumph and Tragedy of J. Robert Oppenheimer (2005) by Kai Bird and Martin J. Sherwin which won the Pulitzer Prize for Biography or Autobiography for 2006. A centennial conference and exhibit were held in 2004 at Berkeley, with the proceedings of the conference published in 2005 as Reappraising Oppenheimer: Centennial Studies and Reflections. His papers are in the Library of Congress.

In a seminar at the Woodrow Wilson Institute on May 20, 2009, based on an extensive analysis of the Vassiliev notebooks taken from the KGB archives, John Earl Haynes, Harvey Klehr and Alexander Vassiliev confirmed that Oppenheimer never was involved in espionage for the Soviet Union. The KGB tried repeatedly to recruit him, but was never successful; Oppenheimer did not betray the United States. In addition, he had several persons removed from the Manhattan Project who had sympathies to the Soviet Union. Haynes, Klehr and Vassiliev also state Oppenheimer "was, in fact, a concealed member of the CPUSA in the late 1930s". According to biographer Ray Monk: "He was, in a very practical and real sense, a supporter of the Communist Party. Moreover, in terms of the time, effort and money spent on Party activities, he was a very committed supporter".

As a member of the Board of Consultants to a committee appointed by Truman, Oppenheimer strongly influenced the Acheson–Lilienthal Report. In this report, the committee advocated creation of an international Atomic Development Authority, which would own all fissionable material and the means of its production, such as mines and laboratories, and atomic power plants where it could be used for peaceful Energy production. Bernard Baruch was appointed to translate this report into a proposal to the United Nations, resulting in the Baruch Plan of 1946. The Baruch Plan introduced many additional provisions regarding enforcement, in particular requiring inspection of the Soviet Union's uranium resources. The Baruch Plan was seen as an attempt to maintain the United States' nuclear monopoly and was rejected by the Soviets. With this, it became clear to Oppenheimer that an arms race was unavoidable, due to the mutual suspicion of the United States and the Soviet Union, which even Oppenheimer was starting to distrust.