United States and weapons of mass destruction
The United States is known to have possessed three types of weapons of mass destruction: nuclear weapons, chemical weapons, and biological weapons. The U.S. is the only country to have used nuclear weapons in combat, when it detonated two atomic bombs over the Japanese cities of Hiroshima and Nagasaki during World War II. It had secretly developed the earliest form of the atomic weapon during the 1940s under the title "Manhattan Project". The United States pioneered the development of both the nuclear fission and hydrogen bombs (the latter involving nuclear fusion). It was the world's first and only nuclear power for four years (1945–1949), until the Soviet Union managed to produce its own nuclear weapon. The United States has the second largest number of deployed nuclear weapons in the world, after Russia.
Nuclear weapons of the United States
In 2009 and 2010, the administration of President Barack Obama declared policies that would invalidate the Bush-era policy for use of nuclear weapons and its motions to develop new ones. First, in a prominent 2009 speech, U.S. President Barack Obama outlined a goal of "a world without nuclear weapons". To that goal, U.S. President Barack Obama and Russian Prime Minister Dmitry Medvedev signed a new START treaty on April 8, 2010, to reduce the number of active nuclear weapons from 2,200 to 1,550. That same week Obama also revised U.S. policy on the use of nuclear weapons in a Nuclear Posture Review required of all presidents, declaring for the first time that the U.S. would not use nuclear weapons against nonnuclear, NPT-compliant states. The policy also renounces development of any new nuclear weapons.