In 2010, North Korea sank a South Korean warship, killing more than forty sailors. The bold attack, conducted in secret by submarine, was the deadliest incident between the two countries in decades. Although Pyongyang’s involvement was suspected from the outset, an international commission set up to investigate the attack later conclude North Korea was indeed responsible.

On the night of March 26, 2010 the Republic of Korean Navy corvette ROKS Cheonan was on patrol off the coast of Baengnyeong Island. The Cheonan was a Pohang-class corvette, designed for coastal patrol duties. North Korea had instigated several ship-to-ship skirmishes with South Korean naval forces in the West Sea, and the Pohang was a large ship capable of decisively taking on smaller, weaker North Korean gunboats.

South Korea built twenty-four Pohang-class ships, divided between anti-surface and anti-submarine warfare variants. Cheonan, an anti-submarine ship, was 289 feet long, displaced 1,200 tons and had a typical crew of ninety-five. Her armament included a bow-mounted Oto Melara 76mm rapid fire gun, the same kind used by the U.S. Navy on the Oliver Hazard Perry–class guided missile frigates, four Harpoon anti-ship missiles, six Mk.46 anti-submarine torpedoes and depth charges.

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On the day of the incident, ROK Navy’s Second Fleet Command warned the Cheonan that a North Korean submarine and six support vessels had disappeared from the port city of Nampo. Located twenty miles southwest of Pyongyang, Nampo is a major hub of North Korean naval activity, including submarine, gunboat and armed hovercraft bases.

Later that evening, at 9:22pm local time there was explosion at the stern of the South Korean warship. A ROK military sentry on Baengnyeong Island reported observing an “approximately 100-meter-high ‘pillar of white flash’ for two or three seconds. An explosion had torn into the ship approximately nine feet from the room where the ship’s General Electric LM2500 gas turbine was located turbine. The ship split in half within five minutes of the attack.

The attack took the ship’s crew completely by surprise. The explosion killed many sailors outright, while those located away from the explosion were knocked off their feet. Of the 104 officers and enlisted men onboard the ship, forty-six were killed. Investigators later reported fractures and lacerations on the bodies of those killed, but no burns or injuries from fragmentation weapons.

The ROK Navy immediately launched a search and rescue effort, with survivors picked up by nearby ships. The ROKS Sokcho, another Pohang-class frigate also steaming in the area, reportedly fired hundreds of shots at an unknown radar target after the attack. The ROK Joint Chiefs of Staff later announced the target to have been a flock of birds. Air and naval forces, including South Korea’s landing platform, dock ROKS Dokdo, set up a perimeter to recover remains and eventually the ship itself. The U.S. Navy sent a salvage ship, the USNS Salvor, two destroyers and amphibious assault ship to assist in the recovery effort.

Speculation that the sinking was the result of North Korean action was rampant from the outset. The suspicious submarine activity at Nampo was one contributing factor, as was the North’s history of sudden, violent attacks against South Korean forces. A mine laid by submarine, or a torpedo attack, was considered the most likely cause. The missing North Korean submarine and support ships returned to port three days after the attack.

The Cheonan, which had settled in shallow water, was raised and brought back to the 2nd Fleet Headquarters at Pyeongtaek to determine the cause of the sinking. The ship was pieced back together on land. An international team was assembled to investigate the incident, made up of civilian and military experts from South Korea, the United States, Sweden, the United Kingdom and Australia.

The Joint Civilian-Military Investigation Group conclusively laid blame for the sinking on a North Korean torpedo. Fragments of a North Korean CHT-02D heavyweight homing torpedo with “No.1” written on it in Hangul, the Korean alphabet, were discovered after the attack. The torpedo had guided itself to the sound of the Cheonan’s gas turbine engines, a theory supported by the fact that the explosion had occurred next to the gas turbine room. Other damage, including a bent keel and wires on the ship that were cut in the absence of heat, were consistent with the shockwave and bubble effect generated by an underwater torpedo warhead explosion.

The attack on the Cheonan was a classic example of an asymmetric military attack. North Korea, consistently bested in naval gun battles by superior South Korean ships, chose to use submarine torpedoes instead of gunboats to attack the ROK Navy. According to The JoongAng Daily, the Republic of Korea Navy, in its rush to become a blue water, global naval power, shifted focus away from the North Korean Navy and anti-submarine operations in the West Sea were given “comparatively less attention”. This vulnerability was undoubtedly noticed by North Korea and exploited to deadly effect.

North Korea has denied responsibility for the attack, calling the the investigation into its cause a “fabrication orchestrated by a group of traitors”—meaning, the South Korean government. South Korea never retaliated for the deaths of forty-six of its sailors. North Korean artillery forces bombarded the island of Yeonpyeong in November of that year but have been generally quiet since the 2011 rise of current leader Kim Jong-un. The relatively long period indicates Pyongyang is not particularly interested in stirring the pot and instigating a military crisis while it continues to improve its nuclear weapons and missile programs. While we may not see further attacks in the immediate future, it may only be because the North is hard at work at other, far more lethal military options.

Kyle Mizokami is a defense and national-security writer based in San Francisco who has appeared in the Diplomat, Foreign Policy, War is Boring and the Daily Beast. In 2009 he cofounded the defense and security blog Japan Security Watch. You can follow him on Twitter: @KyleMizokami.

Image: Reuters