Blurry Seas, Bloody Wars

To the east, the DMZ ends in the Sea of Japan, called the East Sea by the Koreans, where the maritime boundary is merely a straight extension of the Military Demarcation Line and hardly open to dispute.

Geopolitics are much more complicated on the west coast, where the DMZ ends near the confluence of the Imjin and Han rivers, about 40 km northwest of Seoul. From this point, the Han River estuary or Jogang, an officially “neutral zone” which stretches some 60 kilometers between Paju and the Yellow Sea, marks the border between North and South.

It’s at the mouth of the Jogang that things become really tricky.

Unlike the land border and the Han River estuary, the inter-Korean maritime border in the shallow, island-studded Yellow Sea – called the West Sea by the Koreans – was not demarcated by the 1953 armistice in the absence of agreement between the parties.

To make matters even more complex, the armistice agreement stipulated that five remote islands just off the North Korean coast – Baengnyeong-do, Daecheong-do, Socheong-do, Yeonpyeong-do and U-do – would remain South Korean, as they had been almost all the time since the partition of 1945. North Korea has never laid claim to these islands, even if they considerably hamper its access to the open sea.

But where is the limit of territorial waters?

This question has never been settled. Add the fact that the waters around the five West Sea islands are a particularly coveted fishing zone, and you’ll understand why this area has been the scene of the bloodiest clashes between the two Koreas since the end of the war, and remains extremely volatile today. The recurring artillery duels, naval battles and other skirmishes are sometimes dubbed the “Crab War”, after the sought-after crustacean that inhabits these waters.

I will try to make a long story short…

South Korea did not sign the July 1953 armistice agreement, and its president Syngman Rhee intended to continue attacking the North until it was defeated. To curb his warring ardor, the United Nations commander at the time, American General Mark Clark, is believed to have drawn in August 1953 a line of his own, linking 12 coordinates in the Yellow Sea, which U.N. and South Korean forces were forbidden to cross. It was called the Northern Limit Line (NLL).

This is where things get complicated. First of all, there is no known document in which General Clark set his NLL. Despite an investigation by the CIA, the origin of the 12 coordinates, about which everyone has been arguing for years and which South Korea now considers to be its de facto maritime border with the North, remains embarrassingly unclear.

What is certain is that this unilaterally drawn “maritime border” has no basis in international law of the sea. It’s aim was not to be a border, but to prevent South Korea from continuing the war against the North despite the armistice it didn’t sign.

And yet, for over twenty years, the NLL would be respected by both sides. Even without officially recognizing it, North Korea was very happy with the NLL, which protected it from the warmongering of Syngman Rhee and his successors.

It wasn’t until 1973 that North Korea began to actively contest the NLL, and to multiply its incursions south of it. In 1979, Pyongyang claimed a more southerly “West Sea Military Demarcation Line” with narrow maritime corridors linking South Korea’s mainland to its West Sea Islands.

South Korea maintains that the North has tacitly accepted the existence of the NLL for twenty years, and that it therefore constitutes the de facto maritime border.

North Korea claims never to have agreed or even been notified of the existence of this so-called boundary, and points out that the NLL blatantly violates international law. According to most experts, an international arbitration would have every chance of finding in favor of the North. But the South doesn’t want to hear about it: without the NLL, its five West Sea islands would become all but impossible to defend.

In 2018, the two Koreas agreed to create maritime buffer zones to prevent clashes, but this agreement fell apart in 2024. Then, in February that year, North Korean leader Kim Jong Un declared :

“As the southern border of our country has been clearly drawn, the illegal NLL and any other boundary can never be tolerated, and if the Republic of Korea violates even 0.001 millimeters of our territorial land, air and waters, it will be considered a war provocation”.

In the meantime, the ferries that run several times a day from the port of Incheon, near Seoul, to the West Sea Islands refrain from entering the disputed waters.