A look at North Korea's relations and tensions

Key developments of the north-south tensions in Korean history:

— Sept. 9, 1948: Kim Il Sung establishes the Democratic People's Republic of Korea in the northern half of the Korean peninsula.

— June 25, 1950: North Korea invades the South, beginning the Korean War.

— Jan. 1967: South Korean naval ship patrolling waters near maritime border sinks after being attacked by North Korean artillery. Thirty-nine of the 79 sailors on board are killed, 40 injured.

— Jan. 1968: Thirty-one North Korean commandos storm South Korea's presidential Blue House in failed assassination attempt against President Park Chung-hee. Seven South Koreans are killed.

— Dec. 1969: North Korean spy hijacks and reroutes a South Korean airliner to North Korea and takes hostages. Thirty-nine hostages are freed after more than two months.

— Aug. 1974: South Korea's President Chun Doo-hwan narrowly escapes a bombing in Myanmar that kills 21 people, including several government ministers. Perpetrator claims North Korean ties but Pyongyang denies his claims.

— Nov. 1987: Bombing of South Korean airliner kills 115 passengers and crew. The plane had left Baghdad for Bangkok via transit in Abu Dhabi. Two North Korean spies are found responsible.

— Dec. 1991: Six North Korean patrol boats repeatedly cross the Yellow Sea maritime border over nine days, prompting exchange of fire. South Korea says 20 to 30 North Korean sailors are killed, seven South Korean sailors wounded.

— June 2002: Firefight sinks South Korean boat; six South Korean sailors die during the salvage operation.

— Nov. 2009: South Korea says one North Korean died after a brief exchange of fire between the two sides in the Yellow Sea near the maritime border.

— March 26, 2010: Explosion sinks South Korean naval ship near Koreas' western maritime border, killing 46 sailors. An international investigation later finds North Korea to be responsible for the sinking.

— Nov. 23, 2010: North Korea shoots dozens of rounds of artillery onto a populated South Korean island near their disputed western border, prompting South Korea to return fire and scramble fighter jets.

— Jan. 5, 2011: North Korea calls for unconditional talks with South Korea but Seoul dismisses the offer as insincere and said it is still waiting for an apology for two deadly attacks blamed on Pyongyang.

— Jan. 10, 2011: South Korea and Japan hold military talks on accords that would be their first military agreement since Tokyo's colonial rule of the Korean peninsula ended in 1945. One of the main items on the accords is strengthening defense cooperation by sharing important intelligence, mostly on North Korea.

— Feb. 9, 2011: North Korean military officers abruptly walk out of the first official talks with rival South Korea in months, dashing hopes for easing tensions.

— July 22, 2011: During the Southeast Asian regional security conference in Bali, Indonesia, top nuclear envoys from North and South Korea, smiling as they emerge from a meeting that was the first between envoys from the two nations since 2008, say they are ready to work together to resume stalled disarmament talks.

— July 27, 2011: North Korea demands that that United States sign a peace treaty formally ending the Korean War as a senior North Korean diplomat visits New York to negotiate ways to restart six-nation nuclear disarmament talks.

— Aug. 10, 2011: South Korean marines return fire after North Korea launches artillery shells into waters near the disputed maritime line that separates the two rivals. Both sides' shells land in the water; there are no reports of casualties.

— Aug. 24, 2011: North Korea is ready to impose a moratorium on nuclear missile tests if international talks on its nuclear program resume, a spokesman for Russia's president says after talks with Kim Jong Il.

— Oct. 24, 2011: U.S. and North Korean diplomats open talks in Geneva on Pyongyang's nuclear program, the second direct encounter between the two sides in less than three months.

— Nov. 30, 2011: North Korea says that it is making rapid progress on work to enrich uranium and build a light-water nuclear power plant, increasing worries that the country is developing another way to make atomic weapons. Kim Yong-hyun, a professor at Seoul's Dongguk University, says the North's statement appears to be aimed at applying pressure on Washington and the international community to rejoin the nuclear disarmament talks quickly.

— Dec. 18, 2011: In a "special broadcast" from the North Korean capital, state media says Kim Jong Il has died on a train due to "great mental and physical strain" during a 'high intensity field inspection." South Korea, meanwhile, puts its military on high alert, while people in the streets of Pyongyang break into tears as they learn the news that Kim has died of a heart attack. The United States says it is in close contact with allies South Korea and Japan.

— Dec. 30, 2011: North Korea warns the world there would be no softening on its position toward South Korea's government after Kim Jong Il's death as Pyongyang strengthened his son and heir's authority with a new title: Great Leader.

— Jan. 2, 2012: South Korea's president urges North Korea to use the opportunity of its leadership transition to improve relations, but also warns Pyongyang against any provocations. "If North Korea comes forward with a sincere attitude, it will be possible for us to work together to open a new era," President Lee Myung-bak says in a nationally-televised New Year's message.

— Jan. 9, 2012: The presidents of South Korea and China hold their first summit since Kim Jong Il's death opened the door for major changes in North Korea, where Seoul and Beijing share vital interests as well as strong disagreements.

— Feb. 20, 2012: South Korea conducts live-fire military drills near its disrupted sea boundary with North Korea despite Pyongyang's threat to respond with a "merciless" attack — a threat it did not immediately make good on as it is focusing on internal stability two months after the death of Kim Jong Il, analysts say. North Korea is also days away from its nuclear disarmament talks with the U.S. since Kim's death.

— Feb. 23, 2012: Amid cautious optimism, U.S. and North Korean envoys meet in Beijing for their first talks on dismantling Pyongyang's nuclear programs since the death of Kim Jong Il. The countries were on the verge of a deal to have Washington provide food aid, economic help, and diplomatic concessions if Pyongyang suspended its uranium enrichment program when the agreement was upended by Kim's Dec. 17 death.