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Photographer: Ahn Young-Joon-Pool/Getty Images Photographer: Ahn Young-Joon-Pool/Getty Images

North Korea has kept tensions with South Korea alive with a series of deadly clashes since a war between the adversaries ended without a peace treaty in 1953. Last week North Korean leader Kim Jong Un ordered his troops to be ready for combat and declared a “semi-state of war” along the front line following what South Korea called an exchange of fire across the demilitarized zone. North Korea denied firing.

While some of the clashes led to talks to defuse tensions, they usually produced little progress. Here are 10 of the deadliest incidents that have worsened animosity between the countries since the Korean War ended.

Raid on Presidential Office

In January 1968, 31 commandos disguised as South Korean soldiers or civilians raided the presidential complex in Seoul with the intent to kill then-South Korean President Park Chung Hee, father of current President Park Geun Hye.

The attempt was foiled when South Korean police guarding the presidential complex questioned the commandos, who in turn opened fire. Ensuing firefights killed 27 attackers and left more than 30 South Korean soldiers and civilians dead.

Coastal Infiltration

In October the same year, about 120 North Korean commandos infiltrated cities on South Korea’s eastern coast, killing 18 South Koreans, including soldiers and civilians, before most of the attackers were killed or captured.

The raids fueled tensions that eventually gave way to efforts to prevent aggression. After years of formal and secret negotiations, the governments in 1972 announced a nonaggression pact described as a landmark step toward unification.

North Korean Tunnel

Eight South Korean soldiers died in 1975 while searching a North Korean tunnel discovered under the demilitarized zone when booby traps and mines exploded. The 3.5-kilometer (2-mile) tunnel is one of four built by North Korea discovered since the Korean War ended.

Members of the North Korean military march during a parade commemorating the 65th anniversary of the Korea Worker's Party in Pyongyang, North Korea Photographer: Dieter Depypere/Bloomberg

Ax Murder

In August 1976, North Korean soldiers wielding axes attacked a team of U.S. and South Korean soldiers trimming a tree that blocked sight in the demilitarized zone. Two U.S. Army officers were killed, two were wounded and and five South Korean soldiers were also wounded. Then-North Korean leader Kim Il Sung expressed regret after the murder and kept his troops on standby for combat for more than a year.

Myanmar Explosion

In October 1983 a bomb planted by North Korean agents killed 17 South Korean government officials at a martyr’s mausoleum in Rangoon, Myanmar, minutes before then-South Korean President Chun Doo Hwan was to arrive to lay a wreath. Those killed included Chun’s chief of staff and South Korea’s deputy prime minister.

South Korea refused dialog offered by North Korea for months before agreeing in 1984 to negotiations for fielding a joint athletic team to the Los Angeles Olympics.

Navy Airforce

Passenger Airplane

In November 1987, 115 people died when a time bomb planted by two North Korean agents in a Korean Air passenger plane exploded over the waters off Myanmar. One of the agents said after being captured she had received an order to blow up the plane from then-North Korean leader Kim Jong Il in an attempt to cause chaos ahead of the Seoul Summer Olympics in 1988.

Months after the terrorist attack, Roh Tae Woo took power as South Korea’s president and proposed the two countries seek unification through dialog. By 1991 the two countries agreed to join the United Nations together.

Submarine Infiltration

In September 1996, about two dozen North Korean commandos infiltrated South Korea’s mountainous northeastern province after their submarine ran aground. They eluded searchers for 49 days while killing 17 South Korean soldiers, police officers and civilians. One commando was captured with 24 killed.

Kim Jong Un, top leader of the Democratic People's Republic of Korea Kim Jong Un, center, visiting the Kumsusan Palace of the Sun on the occasion of the 62nd anniversary of the Korean War armistice. Photographer: Xinhua News Agency/Getty Images

Naval Skirmish

A North Korean patrol boat opened fire on a South Korean vessel trying to defend the Yellow Sea boundary between the two countries in June 2002. The fight killed six South Koreans and reportedly many more North Koreans. The battle erupted while South Korea was in a celebratory mood hosting the World Cup together with Japan. It took the two countries less than two months to resume talks between their ministers while they continued to push forward with economic cooperation.

Warship Sinking

In March 2010 the South Korean warship Cheonan sank near a Yellow Sea boundary, killing 46 sailors. South Korea concluded after a multinational investigation that a North Korean submarine torpedoed the ship. North Korea has denied involvement in the sinking while South Korea cut off almost all trade with North Korea.

Defense officials of the two countries met six months after the sinking and failed to resolve their dispute over the cause.

Island Shelling

North Korea fired artillery shells at a front-line South Korean island near their disputed Yellow Sea boundary in November 2010, killing two marines and two civilians. South Korea fired dozens of shells in retaliation. North Korea demanded before the firing that South Korea stop live-fire drills near the border.

North Korea walked out of military talks held four months later, blaming South Korea for the artillery battle.

(Updates with deaths under Ax Murder subheadline.)