North Korea's stunning announcement that it had tested a powerful hydrogen bomb would be a dramatic advance in its quest to build a bomb and mount it on a missile that could threaten the U.S. mainland. But South Korea's spy agency thinks the estimated explosive yield from the explosion was much smaller than what even a failed H-bomb detonation would produce. A timeline of some key developments in North Korea's nuclear ambitions:
— Aug. 31, 1998: North Korea fires a rocket over Japan and into Pacific Ocean but its accuracy is reportedly poor with no meaningful strike capability.
— July 5, 2006: North Korea launches a three-stage rocket with a potential range of 6,700 kilometers (4,100 miles) but it fizzles after liftoff, according to U.S. and South Korean officials. North Korea has never acknowledged the launch.
— Oct. 9, 2006: North Korea conducts its first underground nuclear test blast, after citing "extreme threat of a nuclear war" from United States.
— April 5, 2009: A long-range rocket said to be carrying a satellite reaches the Pacific but fails in the third stage. Despite North Korea's claims of success, no satellite reaches orbit, U.S. North American Aerospace Defense Command says.
— April 14, 2009: North Korea quits six-party nuclear disarmament talks and vows to restart nuclear facilities in protest against international condemnation over its rocket launch.
— May 25, 2009: North Korea conducts its second nuclear test.
— June 13, 2009: North Korea says it will start enriching uranium, a possible second route to manufacture nuclear weapons in addition to a plutonium-based program at its reactor.
—May 11, 2010: North Korea claims to have succeeded in nuclear fusion reaction, which led to speculation that the country was trying to build a more powerful bomb. Outside analysts doubt the North's claim.
— Feb. 29, 2012: North Korea announces a moratorium on nuclear and long-range missile tests in food aid deal with U.S.
— April 13, 2012: North Korea launches long-range rocket from west coast that fizzles shortly after liftoff. Pyongyang acknowledges failure. The United States announces it will not carry out the food aid deal.
— Dec. 12, 2012: A long-range rocket launch puts a satellite in orbit in just 10 minutes. The U.S. North American Aerospace Defense Command says North Korea appears to have put object in space.
— Feb. 13, 2013: North Korea carries out its third nuclear test.
— May 9, 2015: North Korea says it has successfully test-fired a newly developed ballistic missile from a submarine. Missiles launched from submerged vessels would be harder to detect that land-based ones, but many analysts say North Korea likely remains years away from having an operational system.
— Dec. 10, 2015: North Korean leader Kim Jong Un claims to have achieved the capability to detonate a hydrogen bomb.
— Jan. 6, 2016: North Korea says it has conducted a hydrogen bomb test.
