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.
2009: In April, North Korea launches a Taepo Dong-2 missile over the Sea of Japan, in violation of Security Council Resolution 1718. The launch prompts council condemnation, and a demand that North Korea refrain from further launches. North Korea withdraws from the Six-Party Talks, and expels international inspectors. On May 25, North Korea tests a second nuclear explosive device. The Security Council adopts Resolution 1874, increasing sanctions on North Korea. In September, Pyongyang announces it has a successful uranium enrichment program. In December, a U.S. delegation goes to Pyongyang for talks. The U.S. and North Korea agreed on the importance of the Six-Party Talks and the need to implement the 2005 Joint Statement, but did not agree on when and how the North Korea will return to talks.
2010: North Korea is blamed for sinking the South Korean naval vessel Cheonan in March. It shells the South’s Yeonpyong Island on November 23. The incidents leave 50 dead. South Korea demands an apology before moving forward in talks with North Korea. Separately, U.S. nuclear scientist Siegfried Hecker tours a North Korean uranium enrichment facility in November and later says he is "stunned" by the sophistication of the facility. He said the facility, with more than 1,000 centrifuges, could be used either to make highly enriched uranium for nuclear weapons, or fuel for an experimental light-water nuclear reactor under construction nearby.
2011: In Senate testimony February 16, Director of National Intelligence James Clapper says North Korea likely has additional undeclared uranium enrichment facilities beyond the facility first revealed in November of 2010. In April, China proposes a three-step revitalization of multilateral talks, beginning with bilateral talks between North and South Korea, followed by similar talks between the United States and North Korea, and, finally, a resumption of the six-party discussions. December 17, North Korean leader Kim Jong Il dies, is succeeded by his youngest son, Kim Jong Un.
2012: In February, the U.S. and North Korea announce agreement by North Korea to suspend operations at its Yongbyon uranium enrichment plant, invite IAEA inspectors to monitor the suspension, and implement moratoriums on nuclear and long-range missile tests. The United States says that it would provide North Korea 240,000 metric tons of food aid under strict monitoring. On April 13, North Korea attempts to launch a weather satellite. It fails. U.S. halts its plans to send food aid to North Korea. December 12, 2012: North Korea launches the Unha-3. The North American Aerospace Defense Command (NORAD) also confirms the launch and says that an object appears to have achieved orbit.
2013: The U.N. Security Council further expands its sanctions against Pyongyang following the satellite launch. North Korea condemns the measures and announces it plans to conduct a third nuclear test. A third nuclear test is conducted, and Pyongyang is struck by more sanctions.
2014: On March 8, China declares a “red line” on North Korea, saying it will not permit war or chaos on the Korean peninsula and that the path to peace can only come through denuclearization. In July, North Korea conducts a series of missile tests ahead of Chinese President Xi Jinping’s visit to Seoul.
2015: In September, North Korea threatens nuclear attack against U.S. and reaffirms its main reactor is operational. In December, Kim Jong Un says North Korea is “ready to detonate” a hydrogen bomb.
2016: January 6, North Korea says it successfully tested a hydrogen bomb.
Sources: U.S. State Department, ArmsControl.org