Origin
The Army of the Potomac was the Union counterpart of the Army of Northern Virginia. It was the major Union force in the Eastern theater of the war, and most often fought against the Army of Northern Virginia. It was founded in 1861, and switched commanders numerous times, mostly because of Lincoln's dissatisfaction with them. The army itself consisted of all military forces in the former Departments of Northeastern Virginia, Washington,Baltimore, and the Shenandoah. Despite its formidable size, the command of the army under less than adept generals caused it to lose many of the battles it fought, including the First Battle of Bull Run, Chancellorsville, and others.
Chain of Command
At the time of the Battle of Gettysburg, the command of the Army was given to George G. Meade just three days before the first day of the battle. The army consisted of six Corps, the 1st, 2nd, 3rd, 4th, 5th, and 6th Corps. At the time just before the battle, the Army consisted of 94,000 men. Though they had suffered some losses at Chancellorsville, they were impressed by Meade's ability.
Gettysburg
At the Battle of Gettysburg, this army fought against the Army of Northern Virginia. During the battle, it was able to constantly repel the Confederate attacks. At Devil's Den and the High Water Mark, the Army of the Potomac was able to defeat the Army of Northern Virginia. However, during the fighting the army sustained casualties of over 24,000 men. However, the Union was able to replace these men, unlike the South.
After Gettysburg
Their victory over the "invincible" Army of Northern Virginia caused the army to realize its capabilities under an adept general. With their morale raised, the Army of the Potomac continued fighting. Participating in a number of victorious campaigns, the Army of the Potomac finally caused Lee to surrender in the Appomattox Campaign. The army was disbanded in 1865.