One month after the southern victory at Chancellorsville in May 1863, the United States War Department informed the governors of northern states that Lee's massive army had moved into the Shenandoah Valley, obviously intent on a northern raid. Pennsylvania Governor Andrew Curtin was rightly concerned over the threat. In early June, the War Department divided his state into two military departments and appointed military commanders for both. The "Department of the Susquehanna" (the eastern department), was placed under the command of General Darius Couch, a veteran officer from the Army of the Potomac. General Couch only had a handful of troops and equipment at his disposal and used his authority to issue orders for volunteers to immediately enlist for the state's defense. Governor Curtin followed with a proclamation on June 12th asking for able bodied men to volunteer for military service in "emergency" militia regiments. Three days later, President Lincoln called for 100,000 men from Pennsylvania, Ohio, Maryland and West Virginia to serve for a period of six months or as long as necessary during the emergency.

Pennsylvania was asked to provide half of the president's quota and by mid-July the commonwealth had raised nearly forty regiments of militia (most were under strengthened) and all were unprepared to be soldiers. Additional time was necessary to gather these newly organized regiments into one location and be placed where they could do some good. As individual companies of 80 to 100 men slowly filtered into Camp Curtin in Harrisburg, they were immediately put to work building fortifications around the city. Barely two weeks later, some of these first "emergency troops" from Pennsylvania and New York experienced combat in June when a mounted Confederate force under General Albert Jenkins raided Chambersburg, Carlisle, Mechanicsburg and threatened Harrisburg. Two skirmishes near Camp Hill, Pennsylvania, which lies on the Susquehanna River opposite Harrisburg, slowed Jenkins' advance while the newly constructed earthworks and fort could be manned.

The 26th Pennsylvania Emergency Militia was the only organized emergency regiment to see battle in the Gettysburg area, which occurred on June 26. General Jubal Early's troops were marching from the Cashtown area toward Gettysburg when his cavalry force, the 35th Virginia Battalion, stumbled upon the green Pennsylvania troops that had been marched out of Gettysburg to stop the southern advance. The 26th's commander, Colonel William Jennings, knew that his men were without training and could not stand up against any veteran force, so he ordered an orderly withdrawal that soon became a rout. The undisciplined troops turned into a mob, clogging the narrow road as they fled with the 35th Virginia hot on their heels- nabbing over 175 officers and men as prisoners of war. Another force scattered a handful of new militiamen in Gettysburg and killed Private George Sandoe of the 21st Pennsylvania Emergency Cavalry as he tried to escape. The poor showing of these untried troops only added confidence to the Confederates who continued the march with little or no additional harassment.

It was not until the Battle of Gettysburg had ended when the majority of the emergency regiments in Pennsylvania were finally organized and mustered into service. Most of these regiments were sent to guard railroads, bridges and fords over major rivers, and to protect state and Federal property located throughout Pennsylvania. Two regiments of militia were sent to Adams County after the battle to help with the recovery process. Colonel H. C. Alleman, commanding the 36th Pennsylvania Emergency Militia, was appointed Military Governor of the district that embraced the battlefield area and all of the hospital sites in the county. They were soon joined by the 51st Pennsylvania Emergency Militia, under Colonel Oliver Hopkinson, which camped on East Cemetery Hill. Soldiers were divided into details and given daily assignments including the priority of recovering government property. Soldiers scoured the fields and farms for abandoned equipment. They even entered private homes because many civilians had taken military equipment from the battlefield as souvenirs. Before the 36th regiment left Gettysburg on August 8, Colonel Alleman reported that his troops collected "Twenty-six thousand six hundred and sixty-four muskets, nine thousand two hundred and fifty bayonets, one thousands five hundred cartridge boxes, two hundred and four sabres, fourteen thousands rounds of small arms ammunition, twenty six artillery wheels, seven hundred and two blankets, forty wagon loads of clothing, sixty saddles, sixty bridles, five wagons, five hundred and ten horses and mules, and six wagon loads of knapsacks and haversacks." All of the gathered stores were transported to the Washington Arsenal for repair and cleaning.

The militia also supervised the removal of wounded to Camp Letterman and then to permanent hospitals via the railroad. 12,061 wounded Union soldiers were sent from field hospitals to hospitals in Baltimore, Philadelphia, Harrisburg, and other northern cities. 6,197 Confederate prisoners were also sent to northern hospitals before transfer to prisoner of war camps. Additionally, 1,637 "stragglers" were also detained and sent to military authorities for return to the army.

Though it was two great armies that fought and decided the Battle of Gettysburg, it was the Emergency Troops that restored order to the state.