During a fierce thunderstorm on the evening of July 4, 1863, a nine-mile-long wagon train of supplies, livestock and wounded soldiers made its way from Fairfield through Monterey Pass. This was the advance of the Confederate retreat column from Gettysburg. That same night Union Cavalry met a local 12-year-old girl, Hetty Zeilinger, walking on the road to her home near Monterey Pass. Hetty warned them that Confederates with artillery were blocking the pass. The brave girl offered to guide the troopers through the pass, around the Confederate forces. One of the soldiers lifted her into his saddle. On they rode, leading a column of Union Cavalry through the darkness and rain, seeking to harass the Confederate's rear and flanks as they withdrew back toward the Potomac.
Approximately 10,000 soldiers fought around Monterey Pass. The battle lasted more than six hours, as they struggled in some of the most bewildering combat of the war. In the lightning, gunfire, and confusion, many of the terrified teams of horses and mules bolted down the side of the mountain, crushing the occupants of the wagons. More than 1,500 Confederates were taken prisoner. Next to Gettysburg, Monterey was the largest Civil War action fought in Pennsylvania.