. With Bataan on the brink of falling, Captain John H. S. Dessez, the commander of the Navy Base at Mariveles requested permission to transfer his 500 sailors to Corregidor. Approval was granted with the condition that these men would be part of the beach defense force. On 10 April, the 4th Battalion was formed under the command of Major Francis H. "Joe" Williams. His command was built with nine Army officers and 16 Navy officers and warrant officers commanding 272 enlisted men. This joint service battalion bivouacked in Government Ravine near Battery Geary and began to train for ground combat.

Four companies were organized in the battalion and lettered Q, R, S, and T. Companies Q and R were commanded by Army officers and S and T by Navy officers. Rifles still packed in cosmoline, a greasy protective coating, were issued to the sailors. This presented interesting cleaning problems to the inexperienced mariners. However, rifles were all that was issued to the battalion in the way of equipment. There were no helmets, cartridge belts, or even first aid kits.