In the fall of 1940, air base squadrons expanded into groups and were assigned materiel squadrons for maintaining aircraft and air base squadrons to provide administrative support, and the unit became the 20th Air Base Group. The 20th Group's station at Nichols Field came under Japanese air attack as World War II began in the Pacific. Although Far East Air Force had built up to the largest American overseas force in terms of personnel, much of its equipment was en route when the Japanese attacked. As the war began, the group supported the 2d Observation Squadron, 17th Pursuit Squadron and 34th Pursuit Squadron at Nichols. Three days after the start of the war, 10 December 1941, saw the first concentrated Japanese attacks on Nichols. By the end of the attack all observation airplanes at Nichols had been destroyed or rendered unserviceable. By 24 December, all bombers had been flown to Australia and air force headquarters and all combat units except for a few fighters soon followed. By the end of December, all American forces were being withdrawn to Bataan. Colonel Harold H. George, of 5th Interceptor Command assumed control of all Air Corps units remaining in the Philippines. With the flying mission being performed by a handful of fighter pilots, the bulk of the group's personnel were employed as infantry. By May 1942, resistance in the Philippines had ended and the group ceased to exist as an organized military formation.