After General Hooker learned of his resignation had been accepted after midnight on June 28th, General George Meade was appointed commander of the Army of the Potomac. Not liking the layout of the Army of the Potomac on South Mountain as well those encamped near Frederick; General Meade started withdrawing his forces off of South Mountain and ordered them to Frederick. Later that day, General Meade issued his marching orders to march northward to Emmitsburg, Taneytown and Union Mills. He wasn't sure of General Lee's intentions as far as attacking York and Baltimore or swinging back down the valley and maybe attacking Frederick and on to Washington. The worst possible scenario for General Lee would be to make a stand at South Mountain in Pennsylvania. By ordering the Union Army northward, General Meade would have control of every road leading to Baltimore, Frederick or even Washington.

By June 28, General James Longstreet and Ambrose P. Hill's corps were at Chambersburg, while the divisions of General Richard Ewell's corps had crossed South Mountain and were located at York and Carlisle, and were preparing to move against Harrisburg. The next day on the 29th, while in Chambersburg, Lee had learned that the Army of the Potomac was at Frederick, and that General Joseph Hooker had resigned and was replaced by General George Meade. General Lee decided to bring his entire army to the eastside of South Mountain and if given the offer, he would fight it out there. General Lee also issued orders to General Ewell to concentrate his corps and move them southward toward Gettysburg or Cashtown.

Since most of General Ewell’s Corps was already at their attended destinations, General Hill began to move some of his divisions to Cashtown. General Henry Heth’s Division would encamp at and around Cashtown on the eastern edge of South Mountain. South Mountain acting as a barrier, the rest of General Hill’s Corps as well as the divisions of General Longstreet’s Corps would be forced to encamp of the western side of South Mountain.

As General Lee was on the march, General George Meade began to move his army north, on June 29th, General Meade ordered his army as follows: The First and Eleventh Corps were to cover the mountain areas marching along the foot of the Catoctin Mountain to Emmitsburg and then into Pennsylvania. The Second Corps was to march to Uniontown. The Third and Twelfth Corps were to march to Taneytown sending the Twelfth Corps to Bruceville. The Fifth Corps was to march to Libertytown and the Sixth Corps was to march to New Windsor. The First and Second Cavalry Brigades of the First Cavalry Division was to move from Frederick to Monterey Pass and then onto Fairfield. The Reserve Cavalry Brigade of the First Cavalry Division was to march to Thurmont. The Second Cavalry Division was to march to New Windsor and the Third Cavalry Division was to march from Frederick to Littlestown and the Artillery Reserve was to march from Frederick to Bruceville. General Meade himself would encamp at Middleburg that night.

Private M. S. Schroyer of the 147th Pennsylvania recalled: "While passing thru Frederick we first learned that General Hooker had been relieved of command of the Army of the Potomac. While marching along rumors were afloat that General McClellan, the idol of the Army of the Potomac, was again recalled to take command. Cheer after cheer was given for Little Mack, as he was familiarly called. Every one seemed to look to him to lead the army on to victory. We learned later on of the appointment of General Meade. All knew of General Meade's sterling worth as a soldier and all had confidence in him as a leader. The entire army had concentrated at Frederick City and so far as we know General Hooker's plans were not changed in the least by General Meade."

Lt. Colonel Rufus R. Dawes of the 6th Wisconsin Infantry wrote about the Union advance toward Pennsylvania: "We left South Mountain in great haste on the 28th and marched to Frederick through a drizzling rain as usual. Next day we moved from Frederick to Emmitsburg, Md., and today we came here, where we are having a muster for pay. I don't think I ever before saw at this time of the year such a long continued, misty, drizzling storm as we have been marching through since we crossed the Potomac. General Meade as commander of the army was a surprise."

Lieutenant William Wheeler of the 13th New York Battery also reflected on the movements of the Army of the Potomac. "The artillery took a road for itself that day, in order not to be encumbered by the infantry, and we made a march of about thirty miles to reach Jefferson City, where we camped in long, wet grass, exposed to a heavy rain storm. The next morning my Battery marched with one brigade to Burkettsville which lies at the foot of South Mountain, and was the scene of the battle of that name in last September. We passed through Frederick after nightfall, and did not see the place; the next day we marched to Emmitsburg and rested there, preparatory to the approaching conflict."

The Federal cavalry and Battery A of the 2nd U.S. Artillery under John Buford moved toward Pennsylvania investigating the Confederate forces in the area. General Buford left Middletown and took the National Pike to Boonsboro, Maryland where he then took the road to Smithsburg; from there he traveled up to Monterey Pass. As General John Buford stood at the opening of Monterey Pass through South Mountain, overlooking the Cumberland Valley, he saw the dust in the background toward the mountains in the Greencastle area. At this time it was evident to General Buford that a major battle would soon erupt in south-central Pennsylvania.

From Monterey Pass, General Buford traveled to Fountain Dale and that evening using the highest point, he looked down the valley toward Fairfield, and saw the campfires of those troops belonging to General Henry Heth's Division of General A.P. Hill’s Corps. From there, General Buford counter-marched back to Fountain Dale and traveled toward Emmitsburg where, he would inform General John Reynolds of the situation.

As General Hill’s Corps began to move, the march over South Mountain at Cashtown Gap was hard. With so many troops of Hill’s Corps on the eastern side of South Mountain, General Longstreet was not able to get his entire corps across, leaving General George Pickett’s Division who was still in Chambersburg behind the wagon train of General Ewell’s Corps in which they were guarding. The next day, both armies were converging upon Gettysburg and a battle was set to commence the next day. General Longstreet finally managed to get his corps across South Mountain, was able to start deploying troops at Gettysburg during the evening of July 1st. Some of his corps such as Pickett’s Division did not reach the battlefield until July 2nd.

During the battle of Gettysburg, General Wesley Merrit and his cavalry were ordered near Thurmont, Maryland (Called Mechanicstown during the Civil War), after being ordered to guard Harman's Pass on the Catoctin Mountain and watching Wolf's Tavern Pass upon South Mountain. These soldiers were the Army of the Potomac's U.S. Cavalry. The U.S. Cavalry was to guard and to protect the roadways and communication lines in the vicinity of Mechanicstown. Its duty was also to guard the Army of the Potomac's supply wagons consisting of an aggregating ten thousand four hundred wagons. A dispatch came to General Merrit on July 2nd to move forward with the wagon train to Emmitsburg, Maryland. General Merrit then received orders to meet with General Judson Kilpatrick on the battlefield of Gettysburg that night.