Marine defense battalions were United States Marine Corps battalions charged with coastal defense of various naval bases in the Pacific during World War II. They maintained large anti-ship guns, anti-aircraft guns, searchlights, and small arms to repel landing forces.
The defense battalions were first conceived from the fixed defense concept during the Marine Corps's, as well the United States Navy's, critical change in their traditional sea service role to a more 'aggressive' amphibious landing force. They conducted "fixed" defense exercises on Culebra Island of Puerto Rico throughout the first half of the 20th century, and other areas around the Caribbean.
The first battalions were created in 1939, when the outbreak of World War II caused concerns that overseas bases might be attacked by the Imperial Japanese Navy.[2] After the attack on Pearl Harbor, where defenders shot down three planes on December 7, 1941, the battalions grew rapidly. On December 8, the Japanese began an assault on Wake Island, and the defenders surrendered after a prolonged battle on December 23.