Squad = 10 men
Platoon = 40 men
Company = 200 men
Battalion = 1,000 men
Regiment = 3,000 men
Division = 20,000 men
Corps = 50,000 or more men
In the Marine Corps, four squads make up one platoon; four platoons and a headquarters section make up a company; three companies and a headquarters company make up a battalion; three battalions make up a regiment; and three regiments plus an artillery regiment make up a division.
A Marine division’s strength is usually about 20,000 men because, in addition to the “line” (infantry) and artillery regiments, there are many special units attached to the division. These special units include battalions of tanks, engineers, motor transport, and amtracks. In addition there are medical-aid, war-dog, rocket, communication, and intelligence units. Two or more of these big 20,000-man divisions form a corps of upwards of 50,000 men, because a corps also has its specialists of service and supply.
At Iwo jima the Fifth Amphibious Corps included three Marine divisions, the 3rd, 4th, and 5th. In all, this was a force of about 70,000 men.
These figures serve only as approximations, because military units always vary in size. A number of men are always sick, on leave, on loan to other units or going home after being discharged. In one campaign a division may need its tanks; in another, impassable terrain will make it necessary to leave the tanks behind. Thus an army, like the wars it fights, is an inconstant, changing thing.