The Northern Limit Line (NLL) is a maritime border between North and South Korea, which was drawn at the end of the 1950-53 Korean War by the United Nations Army Commander as the US-led United Nations Command (UNC) and North Korea failed to produce an agreement on a maritime border due to differences (U.N. claimed 3 nautical mile while North Korea claimed 12 nautical miles) in the armistice negotiations.
The NLL has been enforced by the US and only South Korea has observed it as a de facto sea border since the Armistice Agreement on July 27, 1953. However, North Korea has not recognized it and demanded the creation of a new maritime border through dialogue and negotiation with the US (not South Korea). Since its original establishment was not based on the agreement between the UNC and the North, it has the potential to easily trigger border conflicts between the North and the South. The worst scenario any naval clash in the NLL area can create is the Second Korean War, a full-scale war between the two Koreas since they are technically at war due to the fact that a permanent peace treaty was never signed following the 1950-53 Korean War.