On 3 May 1861 Abraham Lincoln issued a second proclamation calling for 42,034 volunteers for three years' service; expanding the regular army by eight infantry, one cavalry, and one artillery regiments; and expanding the navy by 18,000 sailors. Within four months of the attack on Fort Sumter, the Federal Army grew from 16,000 men to 500,000. Neglected by the War Department, the 19 infantry regiments of the Regular Army never attained their full authorized strength during the war. Instead, the creation of state volunteer regiments afforded state governors opportunities for political patronage in the appointment of regimental officers.
After the war there was a relatively minor reorganization of the Army. Several white regiments were added, and more importantly, four black regiments were created; the 9th and 10 Cavalry and the 24th and 25th Infantry. On 31 December 1860 the United States Army consisted on paper of 18,093 officers and men, with only 16,367 present for duty. The reorganization almost doubled the effective size of the Army. In 1865 and 1866 volunteer units, including some of the black ones, were kept on active duty to police reconstruction, and along the Mexican border [50,000 men] as a none to subtle hint to Napoleon III that he should remove his forces supporting Maximillian from Mexico.
Regards,
Don Dixon