What development has led many world leaders to fear that a nuclear arms race had started

The US government's decision to develop a hydrogen bomb, first tested in 1952, committed the United States to an ever-escalating arms race with the Soviet Union. The arms race led many Americans to fear that nuclear war could happen at any time, and the US government urged citizens to prepare to survive an atomic bomb. Their reason for this panicked prognosis was the United States' decision to develop and test a hydrogen bomb, or H-bomb, a nuclear weapon one thousand times more powerful than the atomic bomb that had leveled Hiroshima at the end of World War II.