The Philippines Campaign or the Battle of the Philippines was the invasion of the Philippines by Japan in 1941–1942 and the defense of the islands by Filipino and United States forces. The defending forces outnumbered the Japanese invaders by 3 to 2, but were a mixed force of non-combat experienced regular, national guard, constabulary, and newly created Commonwealth units; the Japanese used their best first-line troops at the outset of the campaign. The Japanese 14th Army also concentrated its forces in the first month of the campaign, enabling it to swiftly overrun most of Luzon. The Japanese high command, believing they had won the campaign, made a strategic decision to advance by a month their timetable of operations in Borneo and Indonesia, withdrawing their best division and the bulk of their airpower in early January 1942. This, coupled with the decision of the defenders to withdraw into a defensive holding position in the Bataan Peninsula, enabled the Americans and Filipinos to successfully hold out for four more months.