The island's Japanese garrison was composed of the IJN 65th Guard Unit (2,000 men), Japan Navy Captain Shigematsu Sakaibara and the IJA units which became 13th Independent Mixed Regiment (1,939 men) under command of Col. Shigeji Chikamori. Fearing an imminent invasion, the Japanese reinforced Wake Island with more formidable defenses. The American captives were ordered to build a series of bunkers and fortifications on Wake. The Japanese brought in an naval gun which is often incorrectly reported as having been captured in Singapore. The U.S. Navy established a submarine blockade instead of an amphibious invasion of Wake Island. The Japanese-occupied island (called Ōtorishima (大鳥島) or "Big Bird Island" by them for its birdlike shape) was bombed several times by American aircraft; one of these raids was the first mission for future United States President George H. W. Bush.