Preventing the further proliferation of nuclear weapons has been a major goal of U.S. policy ever since the United States developed and used the atomic bomb against Japan in 1945. This goal has taken on new importance for U.S. national security in light of several post-Cold War events. One is the breakup of the Soviet Union; some of the new republics were reluctant to give up nuclear weapons stationed on their territory, and disorder in Russia raised questions about the safety and security of the former Soviet arsenal. The United States is providing assistance to prevent former Soviet nuclear weapons or the materials used to make them from falling into the wrong hands. Another post-Cold War nonproliferation event was Iraq's success in clandestinely developing nuclear weapons despite being a member of the Non-Proliferation Treaty (NPT). Third, North Korea apparently diverted plutonium to a secret bomb program, threatened to withdraw from the Treaty, and continues to block inspections. Other events tend to strengthen nonproliferation efforts. These include the agreement by Argentina and Brazil to allow inspection of their nuclear activities; South Africa's joining the NPT after dismantling six nuclear weapons and agreeing to international inspection of all of its nuclear activities; France's and China's joining the Treaty as nuclear weapons states; and successful enforcement of nonproliferation commitments in Iraq.