The Union Army or Federal Army was the land force that fought for the Union during the American Civil War, 1861 to 1865. It included the permanent regular army of the United States, which was augmented by massive numbers of temporary units consisting of volunteers as well as conscripts. The Union Army fought and defeated the Confederate Army during the war. At least two and a half million men served in the Union Army; almost all were volunteers. About 360,000 Union soldiers died from all causes; 280,000 were wounded and 200,000 deserted. 

When the American Civil War began in April 1861, there were only 16,000 men in the U.S. Army, and of these many Southern officers resigned and joined the Confederate army. The U.S. Army consisted of ten regiments of infantry, four of artillery, two of cavalry, two of dragoons, and three of mounted infantry. The regiments were scattered widely. Of the 197 companies in the army, 179 occupied 79 isolated posts in the West, and the remaining 18 manned garrisons east of the Mississippi River, mostly along the Canada–United States border and on the Atlantic coast. 

With the Southern slave states declaring secession from the Union, and with this drastic shortage of men in the army, President Abraham Lincoln called on the states to raise a force of 75,000 men for three months to put down the insurrection. Lincoln's call forced the border states to choose sides, and four seceded, making the Confederacy eleven states strong. The war proved to be longer and more extensive than anyone North or South had expected, and on July 22, 1861, Congress authorized a volunteer army of 500,000 men.
How many men were in the army at the beginning of the battle?
At least two and a half million