The correct answer is the detonation of a bomb on Russian soil.
In 1949, the Americans were astonished to see that the U.S.S.R. had detonated their own atomic bomb (as a test) in their grounds. It had been only 4 years since the detonation of the atomic bombs of Hiroshima and Nagasaki and they considered it could not be sufficient time to develop the atomic bomb by themselves. Suspicion of espionage was their main option. Time proved them right. Nearly a dozen Soviet spies were convicted of passing information to the Soviets during this period about the atomic bomb in what was called the "Manhattan Project", the most famous spy being Emil Julius Klaus Fuchs. After this experience, the United States began to invest a large quantity of money in protecting their secret projects and fighting espionage.
