By December 1941, the Marine Corps’ six Defense Battalions consisted of 5,000 Marines and constituted 20 percent of the total FMF. These six battalions were given sole responsibility for defending the islands of Wake, Johnston, and Midway, and represented the Allies’ first line of defense against any Japanese aggression in the Pacific. Defense Battalions were among the first to embrace the “detachment concept” and task organize as required for the mission on each island. After the attack on Pearl Harbor, where defenders shot down three planes on 7 December 1941, the defense battalions expanded 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.