Final answer:
Julius and Ethel Rosenberg helped the Soviet Union develop the atomic bomb. They were implicated in a chain of arrests starting with the confession of Klaus Fuchs, a British physicist. The Rosenbergs' involvement is believed to have sped up the Soviet atomic program significantly.
Step-by-step explanation:
Julius and Ethel Rosenberg's Involvement in Atomic Bomb Development
The country that Julius and Ethel Rosenberg helped develop the atomic bomb was the Soviet Union. The pair were implicated through a series of investigations beginning with Klaus Fuchs, a British physicist, who admitted to espionage activities for the Soviets. Fuchs implicated Harry Gold, who then implicated David Greenglass, leading eventually to the arrest of the Rosenbergs in 1950. Their case was highly controversial and remains debated among historians, with the Rosenbergs eventually being executed for espionage in 1953. Their involvement is thought to have accelerated the Soviet Union's atomic bomb development by up to eighteen months.
The Manhattan Project was the American effort to develop the atomic bomb during World War II. This monumental undertaking involved collaboration with Great Britain and had sites across the United States, including Los Alamos, New Mexico. Notably, Germany's progress on their own atomic program was hampered by the emigration of leading scientists to countries like the U.S., where they contributed to the Manhattan Project.