America was the world's first nuclear power and remains the only country to use nuclear weapons in war, dropping bombs on Hiroshima and Nagasaki in Japan in 1945.
During the Cold War the US, along with Russia, built up a nuclear arsenal. Although warmer relations – and arms reductions - followed the collapse of the Soviet Union in 1989, the US retains more than 10,000 nuclear warheads. About 8,000 of these are thought to be operational.
Between 1945 and 1992, the US conducted 1,030 nuclear tests. In 1999, Congress voted to overturn the Comprehensive Test Ban Treaty. Recently the US has considered plans to develop a new generation of smaller, tactical nuclear weapons.