In considering the great theatre of the Civil War, attention is naturally directed to the chief contest of all, and the turning-point of the rebellion, the battle of Gettysburg, fought at the beginning of July, 1863. After the victory at Chancellorsville in May the Confederates determined to carry the war northward into the enemy's country. Gettysburg is seven miles north of the southern boundary of Pennsylvania, and over forty miles from the Potomac River. To the westward is the long curving range of the South Mountain, and beyond this the great Appalachian Valley, a continuation of the Shenandoah Valley, crossing Central Pennsylvania in a curve, and here called the Cumberland Valley. In the latter are two prominent towns, Chambersburg in Pennsylvania, and Hagerstown in Maryland, on the Potomac. General Lee, in preparation for the march northward, gathered nearly ninety thousand men at Culpepper in Virginia, including Stuart's cavalry force of ten thousand. General Hooker's Union army, which had withdrawn across the Rappahannock after Chancellorsville, was then encamped opposite Fredericksburg, and one hundred and fifty miles south of Gettysburg. Lee started northward across the Potomac, but Hooker did not discover it for some days, and then rapidly followed. The Confederates crossed between June 22d and 25th, and concentrated at Hagerstown, in the Cumberland Valley, up which they made a rapid march, overrunning the entire valley to the Susquehanna River, and appearing opposite Harrisburg and Columbia. Hooker, being late in movement, crossed the Potomac lower down than Lee, on June 28th, thus making a northern race, up the curving valleys, with Lee in advance, but on the longer route of the outer circle. There was a garrison of ten thousand men at Harper's Ferry on the Potomac, and Hooker asked that they be added to his army; but the War Department declined, and Hooker immediately resigned, being succeeded by General George G. Meade, who thus on the eve of the battle became the Union commander.