When the United States entered the war, the US Office of Strategic Services (OSS) co-operated and enhanced the work of SOE as well as working on its own initiatives in the Far East. Colonel Wendell Fertig in 1942 organized a large guerrilla force which harassed the Japanese occupation forces on the Philippine Island of Mindanao all the way up to the liberation of the Philippines in 1945. After the surrender of Bataan and Corregidor which was the last organized resistance against the Imperial Japanese Army, Filipino guerillas fought the Japanese throughout the war and became a very important force during the liberation of the Philippines. The exploits of these American commanders and Filipino guerillas influenced the later formation of the United States Green Berets.