Nuclear proliferation is the spread of nuclear weapons to additional countries, particularly those not recognized as nuclear-weapon states by the Treaty on the Non-Proliferation of Nuclear Weapons, commonly known as the Non-Proliferation Treaty or NPT. Nuclear proliferation occurs through the spread of fissile material, and the technology and capabilities needed to produce it and to design and manufacture nuclear weapons. In a modern context, it also includes the spread of nuclear weapons to non-state actors. Proliferation has been opposed by many nations with and without nuclear weapons, as governments fear that more countries with nuclear weapons will increase the possibility of nuclear warfare (including the so-called countervalue targeting of civilians), de-stabilize international relations, or infringe upon the principle of state sovereignty. Conversely, supporters of deterrence theory argue that controlled proliferation decreases conflict rates via nuclear peace.
Nuclear weapons were initially researched during World War II, jointly by the United States, United Kingdom and Canada, and separately by Germany, Japan, the Soviet Union, and France. The United States was the first and is the only country to have used a nuclear weapon in war, when it used two bombs against Japan in August 1945. After surrendering, Germany and Japan ceased to be involved in any nuclear weapon research. A nuclear arms race followed, with further countries developing and testing nuclear weapons. The US primarily competed with the Soviet Union, which carried out their first test in 1949. Seven other countries developed nuclear weapons during the Cold War. The UK and France, both NATO members, developed fission and fusion weapons throughout the 1950s, and 1960s, respectively. China developed both against the backdrop of the Sino-Soviet split.
