Infantry, troops that fight on foot. The objective of the infantry is to attack and destroy the enemy on the ground, or to repel an enemy attack and hold ground. It is one of the three arms of modern armies, the others being artillery and armor (tanks). Infantrymen fight with rifles, machine guns, grenades, bazookas, mortars, and other close-combat weapons. During most of history the infantry has been the backbone of armies.

The basic unit of Alexander the Great's armies was the phalanx, massed rows of infantry armed with long spears. The more maneuverable legion was the heart of the Roman armies that conquered and maintained an empire for centuries. Infantry temporarily lost its place as “the queen of battle” during the Middle Ages, when armored knights on horseback dominated European warfare. (Infantry received its name from the unmounted boys who attended the knights.) When intelligently employed, however, archers and pikemen proved effective against cavalry. With the development of firearms, musket-firing infantrymen became an important element of all armies.

By the time of World War I, defensive weapons and tactics had become so effective that mass infantry formations were subjected to horrible slaughter. After the war some experts thought that airplanes and tanks had outmoded infantry, but in World War II the infantry still bore the brunt of the fighting. Trucks and airplanes greatly increased the infantry's mobility, as did the helicopter, first used in combat in Vietnam.