With news that Fort Sumter had been fired upon, President Lincoln sent for Governor Andrew Curtin. Lincoln needed 75,000 volunteers to be called from all loyal states, but he needed men from Pennsylvania as quickly as possible for the defense of the nations capital. Pennsylvania would be required to supply about one-sixth of the total men for Lincoln's initial request. The volunteers would be armed and equipped at the expense of the federal government, but few preparations had been made. The State would have to supply the volunteers and submit a bill to Washington. The volunteers would be needed immediately and would be asked to serve for three months. Ninety days was generally thought to be ample time to repress the rebellion.

The Secretary of the Commonwealth, Eli Slifer, had begun receiving telegraph dispatches before the President had publicly called upon the northern states to furnish troops. Dozens of dispatches came to the capital, promising thousands of men. By the time the President officially called upon Pennsylvania for 13,000 men, 2,000 to be sent to Washington within three days, enough had already offered their services.

Even as the first defenders were leaving Harrisburg, other companies were continuously arriving. All through the morning and into the afternoon they detrained at the railroad station and poured into the city. Curtin knew that these men had to be organized quickly, for they had to be fed and provided with places to stay. Dauphin County had already offered the use of the County Agricultural Fairgrounds and the governor had accepted. Two companies had already been sent that afternoon and had bivouacked there. The camp was first called Camp Union and eventually changed to Camp Curtin.

Nearly 25,000 Pennsylvanians answered the federal request for 13,000 men. Curtin could not let his people rest. He felt that Pennsylvania could not be allowed to lie idle even for a moment. It must take a leading role in resolving the conflict. His solution came in bringing more men under arms. He knew that when the three month men came home more men would be needed to serve the union.

Curtin was looking ahead and anticipated what would be needed. He advocated, therefore, the creation of a reserve force that could be thrust into the breech in times of trouble. The project would require an enormous appropriation of funds and numerous questions to be answered. The details were to worked out by legislative committees whose work would make the creation of a "well regulated military force."

The legislators were equal to this enormous task and in just 16 days had a bill on the governor's desk. The force was to be called the Pennsylvania Reserve Corps, and the lawmakers had appropriated funding sufficient to raise and equip its 15 regiments; 12 regiments of infantry, 1 rifle regiment, 1 calvary regiment, and 1 artillery regiment. Furthermore, they determined that the Reserve regiments should be gathered at four points throughout the commonwealth. One camp would be located in West Chester near Philadelphia, one in Easton on the Delaware River above Bethlehem and below Scranton, one outside of Pittsburgh, and the fourth at Camp Union (Camp Curtin) in Harrisburg. 

Curtin promptly signed the bill creating the Pennsylvania Reserve Corps and recruiting began at once. The task was not difficult. Many of the companies that had volunteered in mid-April at the first call were still willing and anxious to serve. The governor authorized men to recruit companies with the understanding that they would be rewarded for their effort with a commission of some sort. Often these men merely gathered already filled companies and organized them into a regiment, after which they were elected colonel, lieutenant colonel, or major. In any event, the ranks of the Reserve Corps filled quickly, even after it was learned that the men would be required not to serve three months but three years.