During World War II, the United States Military consisted of the U.S. Army – encompassing the Army Ground Forces, Army Air Forces, and Services of Supply (changed to Army Service Forces in March 1943) – and the U.S. Navy, including the U.S. Marine Corps and the U.S. Coast Guard (components of the Navy with regards to military operations, manpower, and budget, though independent in day-to-day operations).
In 1939, the United States had the 17th largest military in the world behind Romania. On December 31, 1941, the strength of the U.S. Army was 1,657,157 personnel. By December 31, 1943, it was 7,582,434 personnel, and by March 31, 1945 it was 8,157,386 personnel. The U.S. Army contained 91 divisions of armed ground forces during World War II. The U.S. Army Chain of Command as organized from largest to smallest consisted of: Army (50,000+ personnel), Corps (20,000 – 45,000 personnel), Division (10,000 to 15,000 personnel), Brigade (3,000 to 5,000 personnel), Battalion (300 to 1,000 personnel), Company (62 – 190 personnel), Platoon (16 to 44 personnel), and Squad (9 to 10 personnel).
The U.S. Navy grew rapidly during World War II, beginning a growth in construction before the outbreak of the war but kicking into high gear during American neutrality. By the time the United States entered into the war in December 1941, the U.S. fleet consisted of 350 ships with another 350 under construction. By 1945 the U.S. Navy had the largest fleet in the world, with 6,768 operational vessels on V-J Day, August 14, 1945, the day the Japanese surrendered.