The arms race between the united states and the soviet union began when

The arm race began when the Soviet developed the atomic bomb.

Explanation:

As World War II ended, the dispute began when the Soviet Union famously tested its first atomic bomb in 1946. The achievement of a nuclear explosion horrified the world. Thus, the arms race began between the Soviet and the United States, known as the Cold War. The conflict between communist and democratic countries started, which lasted for several years.

They started the nuclear weapon test in 1956 around the cold war.