1950s: North Korea begins nuclear research, focused on energy generation, with assistance from Soviet Union.
1959: Plans begin for nuclear activities near Yongbyon.
1974: North Korea joins the International Atomic Energy Agency, leading to international monitoring of its nuclear work.
1985: North Korea joins the Nuclear Non-Proliferation Treaty (NPT) as a non-nuclear weapons state.
1990: North and South Korea begin talks on nuclear weapons.
1992: Talks result in the 1992 Joint Declaration for a Non-Nuclear Korean Peninsula.
1993: North Korea refuses to implement an agreement with the International Atomic Energy Agency for the inspection of the North's nuclear facilities, and says it will withdraw from the NPT. In May, U.N. Security Council Resolution 825 urges North Korea to cooperate with the IAEA and to implement the North-South Denuclearization Statement. The United States opens talks with Pyongyang in June on ending its nuclear weapons programs.
1994: Washington and Pyongyang issue the Agreed Framework, which calls for North Korea to freeze its existing nuclear program and allow IAEA monitoring. The United States will replace Pyongyang's graphite-moderated reactors with light water reactor (LWR) power plants funded by an international consortium (KEDO) and will provide North Korea with fuel oil until the first reactor is built. The two sides agree to move toward normalizing diplomatic relations.
1995: The U.S. eases economic sanctions against Pyongyang after it freezes its nuclear program.
1997: KEDO breaks ground on site for LWR project.
2002: In October, a U.S. delegation confronts North Korea with evidence that Pyongyang is running a secret uranium enrichment program, violating its IAEA obligations, and its agreements with South Korea and the United States. North Korean officials indicate they have such a program, but Washington insists the North must end the program to ensure a continuing diplomatic thaw. KEDO suspends oil shipments and North Korea says it is ending its freeze on its older nuclear facilities at Yongbyon.
2003: North Korea expels IAEA inspectors and resumes reprocessing spent nuclear fuel at Yongbyon, to extract plutonium for weapons use. The U.S. proposes multilateral talks on the dispute. Pyongyang agrees to meet with China and the United States in April. In August, Pyongyang attends the first round of six-party talks with China, Japan, Russia, the United States and South Korea.
2004: In February, the second round of Six-Party Talks is held in Beijing. A third round is held in June, but North Korea refuses to attend a scheduled fourth round in September.
2005: In February North Korea declares it possesses nuclear weapons and is indefinitely suspending its participation in the Six-Party Talks. In March, it declares itself a nuclear weapons state. The fourth round of talks opens in July and on September 19, the six parties release a joint statement saying North Korea pledges to abandon nuclear weapons programs and to return to the NPT. The other parties agreed to provide economic cooperation and energy assistance and to increase diplomatic ties. But the fifth round of talks in November ends inconclusively.
2006: North Korea launches seven ballistic missiles in July, sparking widespread international condemnation. The U.N. Security Council adopts Resolution 1695 demanding that Pyongyang suspend ballistic missile activities. North Korea immediately rejects the resolution. In October, North Korea tests its first nuclear explosive device. The Security Council passes Resolution 1718, condemning North Korea and imposing sanctions on certain luxury goods and weapons trade.
2007: Discussions resume in February, the parties agree on the "Initial Actions for the Implementation of the Joint Statement." North Korea agrees to shut down and seal its Yongbyon nuclear facility in return for oil and other aid. The sixth round of Six-Party talks is held in March. In July, North Korea shuts down the Yongbyon nuclear facility and oil shipments to the North begin. In a September Six-Party meeting, the parties agree on a plan for North Korea to disable its nuclear facilities and provide a complete list of all nuclear programs and facilities by the end of the year.
2008: North Korea gives its declaration of nuclear facilities to the Chinese in June. In October, the U.S. and North Korea agree on verification measures for the North’s nuclear declaration, and the U.S. drops its designation of North Korea as a state sponsor of terrorism.