World War II
The first nuclear weapon was created by the United States of America. During the Second World War and was developed to be used against the Axis powers. Scientists of the Soviet Union were aware of the potential of nuclear weapons and had also been conducting research on the field.
The Soviet Union was not informed officially of the Manhattan Project until Stalin was briefed at the Potsdam Conference on July 24, 1945, by U.S. President Harry S. Truman, eight days after the first successful test of a nuclear weapon. Despite their wartime military alliance, the United States and Britain had not trusted the Soviets enough to keep knowledge of the Manhattan Project safe from German spies: there were also concerns that, as an ally, the Soviet Union would request and expect to receive technical details of the new weapon.
When President Truman informed Stalin of the weapons, he was surprised at how calmly Stalin reacted to the news and thought that Stalin had not understood what he had been told. Other members of the United States and British delegations who closely observed the exchange formed the same conclusion.
In fact Stalin had long been aware of the program, despite the Manhattan Project having a secret classification so high that, even as Vice President, Truman did not know about it or the development of the weapons (Truman was not informed until shortly after he became president). A ring of spies operating within the Manhattan Project, (including Klaus Fuchs and Theodore Hall) had kept Stalin well informed of American progress. They provided the Soviets with detailed designs of the implosion bomb and the hydrogen bomb. Fuchs' arrest in 1950 led to the arrests of many other Russian spies, including Harry Gold, David Greenglass, and Ethel and Julius Rosenberg.
In August 1945, on Truman's orders, two atomic bombs were dropped on Japanese cities. The first bomb was dropped on Hiroshima, and the second bomb was dropped on Nagasaki by the B-29 bombers named Enola Gay and Bockscar respectively.
Shortly after the end of the Second World War in 1945, the United Nations was founded. During the United Nation's first General Assembly in London in January 1946, they discussed the future of Nuclear Weapons and created the United Nations Atomic Energy Commission. The goal of this assembly was to eliminate the use of all Nuclear weapons. The United States presented their solution, which was called the Baruch Plan. This plan proposed that there should be an international authority that controls all dangerous atomic activities. The Soviet Union disagreed with this proposal and rejected it. The Soviets' proposal involved universal nuclear disarmament. Both the American and Soviet proposals were refused by the UN.