The US nuclear arsenal remained roughly unchanged in the last year, with the Defense Department maintaining an estimated stockpile of some 4,480 warheads to be delivered via ballistic missiles and aircraft. Since September 2009, when the United States announced that the nuclear arsenal contained 5,113 warheads, the stockpile has decreased by 633. Most warheads in the current arsenal are not deployed but stored, and many are destined to be retired. Of the approximately 1,740 warheads deployed, roughly 1,590 are on ballistic missiles or at bomber bases in the United States, with another 150 tactical bombs deployed at European bases. The number of US deployed strategic warheads has decreased since September 1, 2016.