George Meade in the context of Eastern theater of the American Civil War

The Battle of Gettysburg (locally /ˈɡɛtɪsbɜːrɡ/ ) was a three-day battle in the American Civil War, which was fought between the Union and Confederate armies between July 1 and July 3, 1863, in and around Gettysburg, Pennsylvania. The battle, won by the Union, is widely considered the Civil War's turning point, leading to an ultimate victory of the Union and the preservation of the nation. The Battle of Gettysburg was the bloodiest battle of both the Civil War and of any battle in American military history, claiming over 50,000 combined casualties. Union Major General George Meade's Army of the Potomac defeated attacks by Confederate General Robert E. Lee's Army of Northern Virginia, halting Lee's invasion of the North and forcing his retreat.

After his success in the Battle of Chancellorsville in Spotsylvania County, Virginia in May 1863, Lee led his Confederate forces through Shenandoah Valley to begin the Gettysburg campaign, his second attempt to invade the North. With Lee's army in high spirits, he intended to shift the focus of the summer campaign from war-ravaged Northern Virginia in the hopes of penetrating as far as Harrisburg or Philadelphia, which he hoped would convince northern politicians to end the war. President Abraham Lincoln initially prodded Major General Joseph Hooker into pursuing Lee, then relieved him of command just three days before the Battle of Gettysburg commenced, replacing him with Meade.

The Gettysburg campaign was a military invasion of Pennsylvania by the main Confederate army under General Robert E. Lee in summer 1863. It was the first time during the war the Confederate Army attempted a full-scale invasion of a free state. The Union won a decisive victory at Gettysburg, July 1–3, with heavy casualties on both sides. Lee managed to escape back to Virginia with most of his army. It was a turning point in the American Civil War, with Lee increasingly pushed back toward Richmond until his surrender in April 1865. The Union Army of the Potomac was commanded by Maj. Gen. Joseph Hooker and then (from June 28) by Maj. Gen. George Meade.

After his victory in the Battle of Chancellorsville, Lee's Army of Northern Virginia moved north for a massive raid designed to obtain desperately needed supplies, to undermine civilian morale in the North, and to encourage anti-war elements. Lee's army slipped away from Federal contact at Fredericksburg, Virginia, on June 3, 1863. The largest predominantly cavalry battle of the war was fought at Brandy Station on June 9. The Confederates crossed the Blue Ridge Mountains and moved north through the Shenandoah Valley, capturing the Union garrison at Winchester, in the Second Battle of Winchester, June 13–15. Crossing the Potomac River, Lee's Second Corps advanced through Maryland and Pennsylvania, reaching the Susquehanna River and threatening the state capital of Harrisburg. However, the Army of the Potomac was in pursuit and had reached Frederick, Maryland, before Lee realized his opponent had crossed the Potomac. Lee moved swiftly to concentrate his army around the crossroads town of Gettysburg.

The U.S. Army Corps of Topographical Engineers was a branch of the United States Army authorized on 4 July 1838. It consisted only of officers who were handpicked from West Point and was used for mapping and the design and construction of federal civil works such as lighthouses and other coastal fortifications and navigational routes. Members included such officers as George Meade, John C. Frémont, Thomas J. Cram, Stephen Long, and Washington Hood. It was merged with the United States Army Corps of Engineers on 31 March 1863, at which point the Corps of Engineers also assumed the Lakes Survey for the Great Lakes. In the mid-19th century, Corps of Engineers' officers ran Lighthouse Districts in tandem with U.S. Naval officers.

In 1841, Congress created the Lake Survey. The Survey, based in Detroit, Mich., was charged with conducting a hydrographical survey of the Northern and Northwestern Lakes and preparing and publishing nautical charts and other navigation aids. The Lake Survey published its first charts in 1852.