Officially, the two Koreas played little or no role in drawing up their West Sea border known as the Northern Limit Line. With the ink still drying from the 1953 ceasefire that ended the Korean War, the task fell in the hands of the United Nations.

The maritime boundary is often referred to as a de facto border because the Korean War armistice is vague about South and North Korea's territorial rights in the disputed waters. Non-violent incursions by North Korea were frequent starting in the early 1970s, brought on by a fishing frenzy during the May-to-June crab season.

Things boiled over in the summer of 1999 as Pyongyang announced it wouldn't accept the post-war sea demarcation after South Korean patrol boats sank an intruding North Korean torpedo boat, killing most of the onboard crew.

In the months to follow North Korea said the UN-defined sea border was invalid and declared the "Chosun Military Limit Line" as Pyongyang's alternative to the NLL, laying vast claim to South Korean waters. In 2000 North Korea issued a mandate threatening to use force without warning if private South Korean vessels and U.S. Navy ships don't use sea routes designated by Pyongyang when heading to South Korean islands abutting the sea border.

The violent maritime confrontation of 1999 was the first of its kind since the Korean War but the bloodiest naval clash was yet to come. In 2002 a West Sea border skirmish between the two navies resulted in the deaths of four South Korean sailors and inflicted heavy casualties on the North Koreans. But since then, through a series of high-level military contacts, the two Koreas have managed to avoid further bloodshed at sea.

Some military experts believe any revision of the West Sea border would pose a serious security threat. They say North Korea's tactics to push the Northern Limit Line further south is a calculated move to gain the upper hand in military strategy.