a tactical military organization composed basically of a headquarters and two or more companies, batteries, or similar organizations and usually commanded by a field-grade officer. The term has been used in nearly every Western army for centuries and has had a variety of meanings. In the 16th and 17th centuries it denoted a unit of infantry forming part of a line of battle and was loosely applied to any large body of men. During the Napoleonic Wars the French developed an army organization in which the regiment was a unit of administration for its battalions serving as fighting units in the field. In this connection, the terms regiment and battalion often were used interchangeably, but in most modern armies the regiment is a higher unit than the battalion.

In the armies of the Commonwealth nations, infantry battalions, usually commanded by lieutenant colonels, are tactical units formed within regiments, the latter being not tactical but administrative parent organizations. The equivalent tactical artillery and armoured units, however, are called regiments. In most military forces the cavalry equivalent and aviation equivalent of the battalion is the squadron.

In the U.S. Army of the early years of the 20th century, a battalion usually numbered from 500 to 1,000 men and was normally commanded by a lieutenant colonel. After World War I the “square” infantry battalion of four companies was superseded by the “triangular” battalion of World War II and the Korean War, usually composed of three rifle companies, a heavy-weapons company, and a headquarters company. By the late 20th century, the typical U.S. Army battalion was a unit of between 800 and 900 officers and men divided into a headquarters company and three rifle companies. Armoured battalions were organized along similar lines. From two to five battalions formed the combat elements of a tactical brigade, and about 10 battalions formed a division.

In the Soviet army the battalion was smaller than its U.S. counterpart. A typical rifle battalion of a rifle division consisted of 370 officers and men organized into three 78-man rifle companies and machine-gun, artillery, mortar, and service units.