75,000 volunteers

On April 15, 1861, at the start of the American Civil War, the President of the United States, Abraham Lincoln, called for a 75,000-man militia to serve for three months following the bombardment and surrender of Fort Sumter. Some slave states refused to send troops against their neighboring slave states, with the result that most such states also declared secession from the United States and joined the Confederate States.

In April 1861 the Regular Army of the United States of America consisted of approximately 16,000 officers and soldiers organized into ten regiments of infantry, four of artillery, two of cavalry, two of dragoons, and one of mounted rifles. These regiments were mostly posted in small forts of company-sized detachments, the majority posted West of the Mississippi River. Following the secession of seven states from December 1860 to the creation of the Confederate States of America in February 1861, many officers and soldiers resigned from the United States Army to join the Army of the Confederacy.

Until the early 20th century, the US relied on calling out militia and volunteers rather than expanding the regular army. However, there were restrictions on the number of men and the length of time they could serve that the President of the United States, as opposed to a State Governor, could summon.

"Sec. 4. And be it further enacted, That the militia employed in the service of the United States, shall receive the same pay and allowances, as the troops of the United States, And that no officer, non-commissioned officer or private of the militia shall be compelled to serve more than three months in any one year, nor more than in due rotation with every other able-bodied man of the same rank in the battalion to which be belongs." – Militia Act of 1795

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