The following is a chronology of events related to North Korea's nuclear development.
Oct. 9, 2006: First nuclear test conducted.
May 25, 2009: Second nuclear test carried out.
Dec. 17, 2011: Leader Kim Jong Il dies.
Feb. 29, 2012: North Korea agrees to implement moratorium on nuclear tests, long-range missile launches, uranium enrichment in exchange for food aid from United States.
April 11: Kim Jong Un named "first secretary" of ruling Workers' Party of Korea.
April 13: North Korea fires rocket, says it "failed to enter its preset orbit."
April 17: North Korea effectively scraps food aid-for-nuclear freeze deal with United States.
Dec. 12: North Korea launches rocket, says it successfully launched "satellite."
Jan. 22, 2013: U.N. Security Council adopts resolution to tighten sanctions against North Korea.
Feb. 12: 3rd nuclear test conducted.
March 7: North Korea mentions right to make preemptive nuclear attack. U.N. Security Council adopts resolution to impose more sanctions.
April 1: North Korea adopts ordinance to step up nuclear weapons development.
March 26, 2014: Rodong medium-range ballistic missiles launched.
March 30: North Korea says not ruling out new form of nuclear test.
Nov. 18: North Korea indicates conducting fresh nuclear test.
Jan. 9, 2015: North Korea tells United States it is ready to suspend nuclear test if U.S.-South Korea military exercise is cancelled.
March 2: Short-range ballistic missiles fired into Sea of Japan.
May 9: North Korea says it test-fired ballistic missile from submerged submarine.
Jan. 6, 2016: North Korea says hydrogen bomb test conducted.