Following the world's first nuclear test by the US in 1945, the former Soviet Union, United Kingdom, France, and China also succeeded in developing nuclear weapons and repeatedly conducted nuclear tests. These tests injured and killed many persons and caused massive global contamination. As a result, the Partial Test Ban Treaty in 1963 banned atmospheric testing but allowed it to continue underground.
With the end of the Cold War, underground nuclear tests were suspended by the Soviet Union in 1990, United Kingdom and the US in 1992, and France and China in 1996. On September 10, 1996, the Comprehensive Test Ban Treaty (CTBT) was adopted by the UN General Assembly. However, because the treaty does not cover computer-simulated tests and tests not accompanied by explosions (sub-critical tests), the US defied the international call for a world without nuclear weapons and conducted sub-critical tests in 1997. It was learned later that Russia carried out similar tests.
India conducted underground nuclear tests on May 1998. Following India's actions, Pakistan, who has a long-standing territorial dispute with India, also carried out the tests. In addition to that, Democratic People's Republic of Korea (DPRK) conducted underground nuclear tests twice on October 2006 and May 2009. Among States, whose ratification is necessary to the entry into force of the CTBT, the three States named India, Pakistan and DPRK, refused to even sign the CTBT, claiming that the Nuclear Non-proliferation Treaty (NPT) and CTBT are unfair treaties because they have merely contributed to the monopoly of nuclear weapons for traditional five nuclear powers. Now the nuclear non-proliferation system is facing with a danger of total collapse.
It is estimated that 2,099 nuclear tests have been conducted around the world so far (as of the end of May 2009).