Cold War: Ethel and Julius Rosenberg are sentenced to death for spying for the Soviet Union.

Julius Rosenberg and Ethel Rosenberg were an American married couple who were convicted of spying for the Soviet Union, including providing top-secret information about American radar, sonar, jet propulsion engines, and nuclear weapon designs. Convicted of espionage in 1951, they were executed by the federal government of the United States in 1953 using New York's state execution chamber in Sing Sing in Ossining, New York, becoming the first American civilians to be executed for such charges and the first to be executed during peacetime. Other convicted co-conspirators were sentenced to prison, including Ethel's brother, David Greenglass, Harry Gold, and Morton Sobell. Klaus Fuchs, a German scientist working at the Los Alamos Laboratory, was convicted in the United Kingdom. For decades, many people, including the Rosenbergs' sons, have maintained that Ethel was innocent of spying and have sought an exoneration on her behalf from multiple U.S. presidents.

Espionage, spying, or intelligence gathering, as a subfield of the intelligence field, is the act of obtaining secret or confidential information (intelligence). A person who commits espionage on a mission-specific contract is called an espionage agent or spy. A person who commits espionage as a fully employed officer of a government is called an intelligence officer. Any individual or spy ring, in the service of a government, company, criminal organization, or independent operation, can commit espionage. The practice is clandestine, as it is by definition unwelcome. In some circumstances, it may be a legal tool of law enforcement and in others, it may be illegal and punishable by law.

The Union of Soviet Socialist Republics (USSR), commonly known as the Soviet Union, was a transcontinental country that spanned much of Eurasia from 1922 until it dissolved in 1991. During its existence, it was the largest country by area, extending across eleven time zones and sharing borders with twelve countries, and the third-most populous country. An overall successor to the Russian Empire, it was nominally organized as a federal union of national republics, the largest and most populous of which was the Russian SFSR. In practice, its government and economy were highly centralized. As a one-party state governed by the Communist Party of the Soviet Union (CPSU), it was the flagship communist state. Its capital and largest city was Moscow.