Territorial sea is the part of the ocean that a nation controls. For centuries, countries had different opinions about who owned the seas. In 1609, Dutch statesman Hugo Grotius said that all nations should have access to the oceans. Most countries wanted territorial rights.

Methods for measuring boundaries included a 1702 recommendation from Cornelius van Bynkershonk. He proposed that a nation had the right to the area that could be protected by cannons located on land.

During the twentieth century, an international law defined territorial sea and three other coastal zones. The 1994 United Nations Convention on the Law of the Sea treaty defined territorial sea as the area extending up to 12 nautical miles (13.8 statute miles) from a nation's shoreline.

A nation's territorial sea consists of the ocean surface, the airspace above the sea, and the seabed below. Within this area, the nation has the exclusive rights to natural resources, as well as rights for fishing, shipping, and navigation.

The U.N. had discussed coastal boundaries since 1958, and nations met numerous times before approving the treaty in 1994. The United States claimed its 12-mile territorial sea in 1988 when President Ronald Reagan signed proclamation 5928.