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 in the atomic bombings of Hiroshima and Nagasaki in 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 before being joined in the "nuclear club" by the Soviet Union. The United States has the second largest number of deployed nuclear weapons in the world, after Russia.