The critical event that initiated the arms race during the Cold War was the detonation of the first Soviet atomic bomb in 1949. Following World War II, the United States initially held a nuclear monopoly, which contributed to its post-war dominance. However, the successful test by the USSR marked a significant turning point, signaling the start of a competitive build-up of nuclear arsenals between the two superpowers. This event escalated tensions, leading to a series of further developments in nuclear capabilities, including the testing of more advanced weapons. The arms race became a central aspect of Cold War dynamics, influencing global geopolitics.