Marine defense battalions Marine defense battalions were United States Marine Corps battalion s 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 . Organization . 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 . History . 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 . 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 . 1942 became a period of defense for the Pacific Theater , and as such , the Marine Defense battalions saw much reinforcement , redeployment , and growth . On June 4 , the Marines at Midway Island fended off a Japanese aerial attack , which contributed to the victory of the naval battle hundreds of miles away . On August 7 , the 3rd Defense Battalion went ashore with the infantry to Guadalcanal and defended the island ( and others in the Solomon Islands ) against Japanese counterattacks during the Battle of Guadalcanal . In the summer of 1943 , elements of the 9th , 10th , and 11th Defense Battalions supported the Army ' s XIV Corps in the central Solomon ' s campaign . Other battalions helped clear the northern islands , including Bougainville , and by the spring of 1944 , they were all taken . In early 1944 , the Marshall Islands became the next target in the Pacific , and the Marine defenders moved in . By summer , they landed in the Mariana Islands , including Saipan , and Guam . During that time , Commandant of the Marine Corps LtGen . Alexander Vandegrift began to scale back on defense battalions . Two were disbanded , and the remaining began to gradually focus on anti - aircraft defense , rather than coastal defense . Only the 6th , the 51st , and the 52nd remained designated as defense battalions , the rest had become anti - aircraft units under Fleet Marine Force , Pacific . In late 1944 , the 2nd , 5th , 8th , and 16th Antiaircraft Artillery ( formerly Defense ) Battalions formed the 1st Provisional Antiaircraft Artillery Group in preparation to invade the Japanese Home Islands . While they did not participate in the Battle of Iwo Jima , they did land on Okinawa in April 1945 . Soon after , the Japanese surrender lead to the disbanding of most of the defense battalions . Marine defense battalions were seen as an ideal platform for integrating African Americans into units with white leaders , since they trained independently and fought in isolated areas . Those recruits slated for defense battalions were trained at the then - segregated Montford Point ( now known as Camp Gilbert H . Johnson , part of the Marine Corps Base Camp Lejeune complex in North Carolina ) . They would then be assigned to the two black defense battalions , the 51st and 52nd .