The United States shares international land borders with two nations:
The Russia – United States maritime boundary was defined by a disputed agreement covering the Bering Sea, Bering Strait, and Arctic Ocean. The International Date Line essentially acts as the de facto border between the two nations; the USA ratified the USSR-USA Maritime Boundary Agreement, but the USSR failed to ratify it before dissolving, and it was subsequently never ratified by Russia. During the winter, travel between Russia's uninhabited Big Diomede Island and Alaska's Little Diomede Island with a population of 110 is theoretically possible, although not legal, on some occasions when ice flowing through the Bering Strait clogs between the two islands.
The Third Border Initiative is an area of foreign policy concerning the Caribbean Sea border between the United States and the Caribbean region.
Insular areas are in a separate customs territory from the 50 States, the District of Columbia, and Puerto Rico (see Foreign trade of the United States).
Agencies
U.S. Customs and Border Protection is responsible for policing the borders and inspecting people and goods being imported.
The United States Coast Guard actively patrols the nation's extensive maritime borders.
The United States Armed Forces and state and local police may also become involved in border enforcement in certain circumstances.