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.
World War III
Of the three nuclear powers in NATO (France, the United Kingdom and the United States), only the United States has provided weapons for nuclear sharing. , Belgium, Germany, Italy, the Netherlands and Turkey are still hosting US nuclear weapons as part of NATO's nuclear sharing policy. Canada hosted weapons until 1984, and Greece until 2001. The United Kingdom also received US tactical nuclear weapons such as nuclear artillery and Lance missiles until 1992, despite the UK being a nuclear weapons state in its own right; these were mainly deployed in Germany.