The Confederate Army of Northern Virginia began its Retreat from Gettysburg on July 4, 1863. Following General Robert E. Lee's failure to defeat the Union Army at the Battle of Gettysburg, he ordered a retreat through Maryland and over the Potomac River to relative safety in Virginia. The Union Army of the Potomac, commanded by Maj. Gen. George G. Meade, was unable to maneuver quickly enough to launch a significant attack on the Confederates, who crossed the river on the night of July 13 into South Mountain through Cashtown in a wagon train that extended for 15–20 miles, enduring harsh weather, treacherous roads, and enemy cavalry raids. The bulk of Lee's infantry departed through Fairfield and through the Monterey Pass toward Hagerstown, Maryland. Reaching the Potomac, they found that rising waters and destroyed pontoon bridges prevented their immediate crossing. Erecting substantial defensive works, they awaited the arrival of the Union army, which had been pursuing over longer roads more to the south of Lee's route. Before Meade could perform adequate reconnaissance and attack the Confederate fortifications, Lee's army escaped across fords and a hastily rebuilt bridge.

The Battle of Gettysburg was a three-day battle in the American Civil War fought between Union and Confederate forces between July 1 and July 3, 1863, in and around Gettysburg, Pennsylvania. The battle, which was won by the Union, is widely considered the Civil War's turning point, ending the Confederacy's aspirations to establish an independent nation. It was the Civil War's bloodiest battle, claiming over 50,000 combined casualties over three days.