Lee crossed the Potomac early in June, after leaving General Stuart with his cavalry and a part of Hill's corps to prevent Hooker from pursuing. He began to concentrate his army around Hagerstown, Md., and prepare for a campaign in Pennsylvania. His army was organized into three corps under the respective commands of Longstreet, Ewell and A. P. Hill. Lee had driven his army so as to enter Pennsylvania by different routes, and to assess the towns along the way with large sums of money. In the latter part of June Lee was startled by the information that Stuart had failed to detain Hooker, and that the Federals were in hot pursuit. He soon conceived that the two armies must soon come together in a mighty death struggle, which meant that a great battle must be fought, a greater battle than this western world has heretofore known, which is claimed by historians as being one of the decisive battles of the world. The Army of the Potomac had changed leaders, and George Gordon Meade was now its commander, having succeeded Hooker on June 28th. Thus for the third time the Army of the Potomac in ten months had a new commander. The two great armies were scattered over portions of Maryland and southern Pennsylvania. Both were marching northward along parallel lines, the Federals endeavoring to stay between Lee's army and Washington. It was plain that they must soon come together in a gigantic conflict; but just where the shock of battle was to take place was yet unknown. Meade sent General Buford in advance with 4,000 cavalry to intercept the Confederate advance guard. On the night of June 30th Buford encamped on a hill a mile west from Gettysburg, and here on the following morning the great battle had its beginning. On the morning of July 1st the two armies were still well scattered, the extremes forty miles apart. 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, with Hill's corps several miles in advance. Buford opened battle against the advance division of Hill's corps under General Heth. Reynolds soon joined and the first day's battle was now in full progress. General Reynolds, while placing his troops in line of battle early in the day, received a death shot in the head by a Confederate sharpshooter. This was a great loss to the Federals, as he was one of the bravest and most able generals in the Union army. No casualty of the war brought more widespread mourning to the North than the death of General John F. Reynolds. But even this calamity did not stay the fury of the battle.