Marine defense battalions

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.

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:

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. In July 1941 the 5th Marine Defense Battalion landed in Iceland as part of the 1st Provisional Marine Brigade alongside the 6th Marine Regiment and other elements of the 2nd Marine Division, augmenting British forces until they departed in September. The battalion had left its Coast Defense Group behind and exchanged the aircraft sound locators for SCR-268 radar sets. In March 1942 the battalion was relieved by the Army's 61st Coast Artillery Regiment. After the attack on Pearl Harbor, where defenders shot down three planes on 7 December 1941, the battalions grew rapidly. On 8 December, the Japanese began an assault on Wake Island, and the defenders, including 399 Marines of the 1st Defense Battalion, surrendered after a prolonged battle on 23 December.

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