From the summer of 1862, Pennsylvania and the District of Columbia were in the theater of war. The populations of adjoining states knew they too might be invaded at any time, at least until the narrow victory at Gettysburg seemed to end the danger. But who knew what to believe? Only ten months before, General Lee had been stopped at Antietam, yet his army had swarmed up the valleys again. These were the states that furnished many volunteers for the Army of the Potomac, and it was
this army that would sustain the highest casualty rates on the Union side throughout the four years of warfare. New York and Pennsylvania, first and second in population, were also first and second in numbers of troops furnished and in casualties taken.
Pennsylvania numbered its infantry regiments as their organization was begun; the last was the 215th, mustered in April 1865 and disbanded that July. New York's highest number was the 194th infantry, mustered after Lee's surrender and expiring within two weeks. A few "paper" regiments in every state never completed their organization at all. Some became so depleted that they were merged with other units. And their number designations could be confusing: there were three 69th infantry
regiments from New York, two of them with distinguished records. The 1st New Jersey Brigade left the state only in August of 1862, but was so seldom out of action thereafter that by the end of the war it had sustained the fourth highest casualties of any brigade
in the Union army. West Virginia, which was only created midway through the war, furnished 32,000 men, of whom 4,000 would not return alive. Units were typically local in origin, and sometimes ethnic as well. New York, for example, provided the Polish Legion, Garibaldi Guard, Irish Rifles, Highlanders, and others. Pennsylvania's 75th Infantry Volunteers was all German.