In late 1862 the Army of the Potomac (AotP) crossed into Virginia with a well devised plan to invest the city of Fredericksburg, and split Robert E. Lee's Army of Northern Virginia in half, thus being in a position to destroy both halves in turn. Major General Ambrose Burnside, commander of the AotP, arrived on the north bank of the Rappahannock River and waited for his pontoon bridges to arrive from the rear. And he waited. And waited. Having arrived there on the 17th of November, the pontoon boats did not arrive until the 25th, a span of over a week that allowed Lee to concentrate his two army corps (under Lieutenant Generals James Longstreet, and Thomas Jonathan "Stonewall" Jackson). Lee's 75 thousand men fanned out in a long line on the heights above the town of Fredericksburg, with Longstreet closest to town, and Jackson's line running south along the river. Burnside failed to move for the next three weeks. His force of 130,000 men, nearly twice the Confederate force, simply sat in camp. Finally, on the 13th of December, Burnside ordered a forced crossing of the river, and an assault on the Confederate positions on Marye's Heights just south of town about a mile from the river. Frontal assaults made by brigades and divisions failed to breach the lines of Longstreet's men, which were supported by over 300 artillery pieces. Most of the Union troops never made it up the hill to Marye's Heights, cut down on the cold, snow-covered ground. Two days later, Burnside retreated, having inflicted just over 5,300 casualties on the Rebels, but suffering well over 12,500 of his own. President Lincoln lost confidence in Burnside, and replaced him with Major General Joseph "Fightin' Joe" Hooker in January.

Hooker took his men into camp, and re-supplied them, re-organized and trained them for the spring offensive. Dumping Burnside's "Grand Division" organization, he set up the Army of the Potomac into the Corps system used at Gettysburg. But first it was tested at the Battle of Chancellorsville, in May of 1863. Crossing the Rappahannock River, Hooker's Army of the Potomac caught Lee up short, with his forces divided. It was a good plan that Hooker had devised, having a large force cross at Fredericksburg where the bulk of Lee's Army of Northern Virginia lay, while Hooker's main force crossed the river 10 miles upstream, and behind Lee's lines. Only the rapid movement of Jackson's force forestalled the expansion of the Union held territory. That night, Jackson moved his men around the Union Army by means of an old logging road through the heavy forest. His men burst from the tangled underbrush at dawn, catching Major General Oliver Otis Howard's XIth Corps making breakfast. The German-Americans of the XIth corps broke the line. Even though hard fighting had steadied the Union lines later in the day, and pressure by the force at Fredericksburg had forced Lee to split his force again, Hooker lost confidence in himself, and his position, and ordered his stunned generals to retreat back across the Rappahannock. Over 17,000 Union casualties were balanced against nearly 13,000 Confederate ones. A costly battle for both sides. Ultimately, it was far more costly for the Confederate Army with the loss of Lieutenant General Thomas Jonathan Jackson. "Stonewall" was wounded severely by friendly fire while reconnoitering the lines on the evening of May 2nd. He died of pneumonia several days later, as a result of his wounds.

Hooker wasn't quite finished. In June of 1863, he anticipated Lee's coming invasion of Maryland and Pennsylvania. On 9 June, he sent a combined force of infantry and cavalry across the Rappahannock River under the command of Major General Alfred Pleasonton, Cavalry Corps Commander for the AotP. Crossing in the early morning darkness, Brigadier General John Buford's Cavalry Division surprised the troopers of Brigadier General William E. "Grumble" Jones Brigade, over-running the cavalry camps, capturing about 150 men. Buford connected with a second brigade of cavalry under Brigadier General David McMinn Gregg, who had crossed the river down stream. Gregg had pushed through the lines of Confederate Brigadier General Beverly H. Robertson's cavalry brigade. Major General James Ewell Brown "Jeb" Stuart rallied his Confederate cavalry and the Battle of Brandy Station was on in earnest. Stuart finally pushed Buford back, when Gregg showed up on his flank. To the south, a third Union cavalry force, a full division under Colonel Alfred Dufie, was engaged in a running battle with several Confederate regiments of cavalry. By this time it was full daylight, and Pleasonton could see the advancing cloud of dust made by Confederate infantry approaching the battle. He ordered his men to withdraw back across the river.

The Battle of Chancellorsville had shown that the Army of the Potomac, in spite of "Fightin' Joe" Hooker, was now a fighting force to be reckoned with, while the Battle of Brandy Station demonstrated that the Union Cavalry Corps was no longer the personal punching bag of Jeb Stuart. The Army of the Potomac had learned its hard lessons, and become a fine fighting force. One of Hooker's last official acts as commander of the AotP was to promote three young cavalry captains to Brigadier General, and assign them brigades to command. One, Wesley A. Merritt took command of a brigade of US Regular Cavalry assigned as a reserve brigade to John Buford's First Cavalry Division. The other two were assigned to Major General Judson Kilpatrick's Third Cavalry Division. These two officers were Elon J. Farnsworth, and George Armstrong Custer. While all three did good service at Gettysburg, Custer wasted no time making a name for himself.

Unable to sustain the confidence of either the army or the president, Hooker asked to be relieved of command. On June 28th, with Lee's men already in Pennsylvania, President Lincoln ordered Major General George Gordon Meade, a Philadelphian, to command of the Army of the Potomac. Meade had worked his way up through the ranks, slowly, steadily, displaying calmness under fire, and a natural ability to adapt to battle changes. His men broke through Jackson's lines at Fredericksburg in December, 1862, in a glorious fight that was unsupported on either flank. The penetration was thus too exposed, and had to be withdrawn. Meade's accession to command was ordered by Lincoln on the recommendation of Major General John Fulton Reynolds, of nearby Lancaster, Pennsylvania. Reynolds was the senior commander, and Lincoln offered command to him. Reynolds preferred to remain with his 1st Corps, and declined the Army command, instead suggesting Meade. Three days before the biggest battle of the Civil War, George Meade assumed command of the Army of the Potomac. One of his first orders was to set his generals minds at rest by informing them there would be no change of commands except to elevate Major General George Sykes to replace him as commander of the Vth Corps. 

Meade also devised a plan of defense in case the army did not concentrate properly, or fast enough. Started north by Hooker around June 23rd, the Army of the Potomac's seven infantry corps (Ist under Reynolds, IInd under Major General Winfield Scott Hancock, IIIrd under Major General Daniel Sickles, Vth under Sykes, VIth under Major General John Sedgwick, XIth under Howard, and XIIth under Major General Henry Warner Slocum) took various routes through Maryland, while scouting reports indicated that Lee was into the northern reaches of the Cumberland Valley with one of his three corps loose east of South Mountain, and ranging across Cumberland, Franklin, Adams, and York Counties, threatening Chambersburg, Carlisle, Harrisburg, York, and Gettysburg. But all roads led to Gettysburg. One look at the map and one can readily see the network of roads converging on that town. Naturally, the troops of both sides began to gravitate toward Gettysburg. Meade's plan centered on a creek just south of the Mason-Dixon line into Maryland. Pipe Creek was a natural place for Meade to block any moves by Lee to head toward Baltimore or Washington. He felt the forces moving toward Harrisburg were a feint to pull him out of position and then unable to protect the nation's capital. The Pipe Creek plan was circulated to his corps commanders.

Meanwhile, Brigadier General John Buford's First Division consisting of two brigades of cavalry and a battery of horse artillery, moved north through the town of Gettysburg on June 30th. There, the fateful decision was made by Buford to defend the ground northwest of town long enough for the Union infantry to arrive and take up positions on the high ground southeast of the town. Buford recognized the importance of the ground from Culp’s Hill, northwest to Cemetery Hill, then south along Cemetery Ridge for over a mile to the smaller of two hills there, called Little Round Top. Buford's keen eye for the ground, and his judgment to make the decision to have the army fight at Gettysburg was soundly reinforced on the morning of July 1st when Major General John F. Reynolds arrived. Reynolds agreed whole heartedly with Buford's assessment. It was especially good ground for inviting an infantry attack, and the Union men knew Lee was without Jeb Stuart and his horsemen. Meade's Pipe Creek plan became the backup plan. 

Brigadier General George Armstrong Custer had been trailing Stuart for several days. Stuart had been ordered to maintain contact with the right of Major General Richard S. Ewell's Second Corps, Army of Northern Virginia as it advanced through the Pennsylvania countryside. But Stuart found himself in a bad position. Hampered by captured wagons loaded with supplies, his men were unable to move with the speed they usually did, and thus allowed nearly the entire Army of the Potomac to march into position between Stuart and Lee. Deprived of Stuart, who was often referred to as Lee's eyes and ears, Lee did not know until the 29th or 30th of June of Meade's promotion. At Hanover, Pennsylvania on the morning of June 30th, George Custer quite rudely introduced himself to Jeb Stuart. Forcibly ejecting Stuart from Hanover, Custer trailed the Rebel Raiders north, in their search for Ewell's infantry. On the night of the first, a messenger delivered orders to Stuart in the town of Dover, York County, Pennsylvania. He was to move to Gettysburg the next day. On July 2nd, Custer hit his column at the village of Hunterstown. And on July 3, he would order his Michigan Brigade to "Ride you wolverines!" on a farm east of the main battlefield, and thwart Stuart's attempt to get into the rear of Meade's lines. (Custer would continue to dominate the relationship for nearly another year until the Battle of Yellow Tavern, Virginia, in May of 1864, where members of Custer's command would kill Stuart.)

The Army of the Potomac's Seven Corps were organized usually into three divisions, each division containing two to four brigades. Each brigade would normally contain about four regiments. The Army of the Potomac contained just over 100,000 men at Gettysburg. The artillery was masterfully commanded by Brigadier General Henry Hunt. During the battle, John Reynolds was killed (on the first day) and was replaced temporarily by Major General Abner Doubleday. Meade them replaced Doubleday with Major General John Newton. Also, Major General Daniel Sickles, Commander of the IIIrd Corps lost his leg early in the fighting on July 2nd. He proceeded to order himself to Washington, taking his leg, and his story with him. He was replaced in corps command by Brigadier General David B. Birney. Major General Winfield Scott Hancock was shot in the groin at the start of the infantry action of July 3rd (Pickett/Pettigrew/Trimble assault), and was replaced in corps command by Brigadier General John Gibbon.