Final answer:
In response to the US atomic bombings of Hiroshima and Nagasaki, the Soviet Union developed and tested its nuclear weapons, contributing to the start of the Cold War nuclear arms race.
Step-by-step explanation:
The Soviet Union responded to the United States' use of atomic bombs on Hiroshima and Nagasaki by developing and testing their nuclear weapons through research and espionage. In the wake of the American monopoly on nuclear technology, the Soviets quickly worked to establish their capabilities. The U.S. development of the hydrogen bomb further spurred the Soviet Union into action, leading them to conduct their successful nuclear tests, starting with an atomic detonation in 1949 and then a hydrogen bomb in 1953. This marked the beginning of the nuclear arms race during the Cold War, as both superpowers amassed vast arsenals of nuclear weapons.