 The island was essential to the invading Japanese forces, as it was the last remaining obstacle to Manila Bay, known as the finest harbor in the Orient.The regiment was composed of troops from 142 different U.S. and Filipino organizations with nearly 4,000 men with the regiment occupied the island at the time, but more than half were Army and Navy personnel without ground combat training.Approximately 1,500 U.S. Army and Philippine Scouts personnel reinforced the regiment during the defense of Corregidor, designated as the Reserve Battalion.