The Army of the Potomac had again changed leaders, and George Gordon Meade was now its commander. Hooker, after a dispute with Halleck, resigned his leadership, and Meade, the strongest of the corps commanders, was appointed in his place, succeeding him on June 28th. The two great armies--Union and Confederate--were scattered over portions of Maryland and southern Pennsylvania. Both were marching northward, along almost parallel lines. The Confederates were gradually pressing toward the east, while the Federals were marching along a line eastward of that followed by the Confederates. The new commander of the Army of the Potomac was keeping his forces interposed between the legions of Lee and the Federal capital, and watching for an opportunity to force the Confederates to battle where the Federals would have the advantage of position. It was plain that they must soon come together in a gigantic contest; but just where the shock of battle would take place was yet unknown. Meade had ordered a general movement toward Harrisburg, and General Buford was sent with four thousand cavalry to intercept the Confederate advance guard. On the night of June 30th Buford encamped on a low hill, a mile west of Gettysburg, and here on the following morning the famous battle had its beginning. On the morning of July 1st the two armies were still scattered, the extremes being forty miles apart. But General Reynolds, with two corps of the Union army, was but a few miles away, and was hastening to Gettysburg, while Longstreet and Hill were approaching from the west. Buford opened the battle against Heth's division of Hill's corps. Reynolds soon joined Buford, and three hours before noon the battle was in progress on Seminary Ridge. Reynolds rode out to his fighting-lines on the ridge, and while placing his troops, a little after ten o'clock in the morning, he received a sharpshooter's bullet in the brain. The gallant Federal leader fell dead. John F. Reynolds, who had been promoted for gallantry at Buena Vista in the Mexican War, was one of the bravest and ablest generals of the Union army. No casualty of the war brought more widespread mourning to the North than the death of Reynolds.