Marine Defense Battalions were United States Marine Corps battalions charged with coastal and air defense of advanced naval bases during World War II. They maintained large anti-ship guns, anti-aircraft guns, searchlights, and small arms to repel landing forces.
Unlike the mobile Marine forces involved in offensive actions, defense battalions were detached to key outposts, in the Pacific and one in Iceland, and remained at the station they defended. Most varied greatly in size and equipment. The battalions often had several coastal gun batteries, several anti-aircraft batteries, a detection battery (searchlights and radar), and machine gun units. While a few had composite infantry companies attached, most defense battalions were responsible for providing their own riflemen.
A 1939 table of organization and equipment (TOE) included:
HQ Company
Service battery
Six platoons, each with a searchlight and aircraft sound locator
Coast Defense Group
Three batteries, each with two Mark 15 5"/51 caliber guns
Antiaircraft Group
Four AAA gun batteries, each with four mobile 3-inch M3 guns
Two AAA machine gun companies, each with 24 Browning M2 .50-caliber machine guns on AA mounts
Two beach protection machine gun companies, each with 24 Browning M1917A1 water-cooled .30-caliber machine guns
It is likely that the 5"/51 caliber guns were replaced by the 155 mm Long Tom and the 3-inch guns were replaced by the 90 mm Gun M1/M2/M3 by early 1943.
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.