The United States developed the first nuclear weapons during World War II in cooperation with the United Kingdom and Canada as part of the Manhattan Project , out of the fear that Nazi Germany would develop them first . It tested the first nuclear weapon on July 16 , 1945 ( " Trinity " ) at 5:30 am , and remains the only country to have used nuclear weapons in war , devastating the Japanese cities of Hiroshima and Nagasaki . It was the first nation to develop the hydrogen bomb , testing an experimental prototype in 1952 ( " Ivy Mike " ) and a deployable weapon in 1954 ( " Castle Bravo " ) . Throughout the Cold War it continued to modernize and enlarge its nuclear arsenal , but from 1992 on has been involved primarily in a program of Stockpile stewardship . The U.S. nuclear arsenal contained 31,175 warheads at its Cold War height ( in 1966 ) . During the Cold War , the United States built approximately 70,000 nuclear warheads , more than all other nuclear-weapon states combined
Ivy Mike
Ivy Mike
List of states with nuclear weapons