Which event marked the start of the nuclear arms race? Soviet Union's first test of an atomic bomb in 1949 United States' first test of a hydrogen bomb in 1952 Soviet Union's launching of Sputnik 1 in 1957 Cuban Missile Crisis of 1962

The event that marked the start of the nuclear arms race was the Soviet Union's first test of an atomic bomb in 1949. Prior to Soviet ownership of nuclear weapons, the US was the only country that had a nuclear arsenal and thus there was no race. Once the Soviets developed their own nuclear program, however, both sides immediately began developing large stockpiles and contingency plans in order to hypothetically overpower the other in the face of war.