Regiment

American Civil War

In the U.S. Civil War, both the Union and the Confederacy raised numerous regiments that included roughly 1,000 men when they took the field. Combat casualties and sickness could quickly attrite these numbers down to as low as 400. In this conflict, the regiment was the essential tactical unit. At the time, the U.S. Army relied on a concept known as the expandable army, under which state units would be recruited to rapidly enlarge a very small peacetime force. In the North, this practice led to competing systems of the U.S. Army, USA, and U.S. Volunteers, USV. Both of these organizations supplied regiments to the Union Army. In the Confederacy, various states raised their own regiments, which were then sent to join the larger field armies. On both sides, units were often designated by the state from whence they were recruited and a number. This practice, along with the expandable army concept, remained in place through the First World War.

World War I

Toward the end of the First World War, the Rainbow Division—so called because it drew from numerous states—was created as a means of creating a more unified national military force. Still, many regimental organizations remained intact. Likewise, the National Defense Act of 1920 greatly modernized the overall army organization. Still, the regiment remained a significant component, even though it was superseded by the division as the main tactical unit.

World War II

During World War II, U.S. infantry regiments were composed of three battalions each. The respective battalions were themselves made up of four companies. Three regiments, in turn, created a division, which contained 15,000 men. During this conflict, regiments were then designated solely by numbers.

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