During the Civil War era, the 195 acres that comprise the River Campus were part of a 1,120-acre plantation owned by Judge Richard Parker, who was best known for presiding over John Brown's trial in the autumn of 1859 and spent time at the Retreat though his primary residence was in Winchester. The house, built in 1799 on the opposite side of the road, survives in private ownership. Both Union and Confederate armies frequently passed through the area, using Snickers Gap as a gateway into the Shenandoah Valley, and Parker's Retreat figured significantly in the Battle of Cool Spring, the largest and bloodiest battle fought in Clarke County. That battle pitted elements of Confederate general Jubal Early's command against a contingent of Union forces led by Gen. Horatio G. Wright; in total 13,000 Union and Confederate troops fought there, and the Confederate victory resulted in more than 800 casualties. On the afternoon of July 18, 1864, approximately 5,000 Union soldiers commanded by Col. Joseph Thoburn marched across Parker's property and crossed the Shenandoah River, and three Union artillery batteries deployed on the heights in the battle's final phase. Afterward, exhausted and wounded Union soldiers sought safety on this side of the Shenandoah, and during the night of the 18th and into the following morning troops from the 2nd Rhode Island Infantry picketed the river's eastern bank and provided cover for wounded Union soldiers escaping there.