The United States holds the second largest arsenal of nuclear weapons among the nine nuclear-armed countries. Under the Manhattan Project, the United States became the first country to manufacture nuclear weapons and remains the only country to have used them in combat, with the bombings of Hiroshima and Nagasaki in World War II against Japan. In total it conducted 1,054 nuclear tests, the most of any country. It is an original party to and one of the five "nuclear-weapon states" recognized by the 1968 Treaty on the Non-Proliferation of Nuclear Weapons.