In 1963, the united states and the soviet union signed a treaty stating that

On August 5, 1963, representatives of the United States, Soviet Union and Great Britain signed the Limited Nuclear Test Ban Treaty, which prohibited the testing of nuclear weapons in outer space, underwater or in the atmosphere. The treaty, which President John F. Kennedy signed less than three months before his assassination, was hailed as an important first step toward the control of nuclear weapons.

Not sure this is a hard one huh