Why is the United Kingdom responsible for Anguilla’s defense?

The United Kingdom is responsible for Anguilla’s defense because Anguilla is a British overseas territory. As part of this constitutional arrangement, the island maintains no military of its own.12

Executive power on the island is held by a governor appointed by the British monarch, who is specifically charged with the responsibility for the territory's external affairs, defense, and internal security. This status was formalized following the Anguilla Act of 1971, which placed the island directly under British control, and subsequent constitutional developments that established its current relationship with the Crown.21