Based on his outstanding performance in the Western Theater, President Lincoln appointed General John Pope to command the Army of the Potomac. Pope immediately had a bad start by issuing a statement to his new army that the performance of Western Theater armies of the Union were superior than those of the Eastern Theater.
I believe General Meade was in command of the Union's Army of the Potomac.
Major General George G. Mead was in command of the Union Army of the Potomac at Gettysburg.
The command of the Federal Army of the Potomac went to Major General Joseph Hooker after Fredericksburg.
On November 7, 1862, General Burnside took command of the Army of the Potomac. He had replaced General George B. McClellan.
Reportedly, Union General Ambrose was offered the command of the Army of the Potomac three times. He reluctantly accepted it and thus replaced George B. McClellan. Ambrose is said to have refused the command because of his loyalty to McClellan combined with the heavy responsibility he would inherit.
As a condition for taking command of the Army of the Potomac, General Joseph Hooker was allowed to report directly to President Lincoln. No other Union field general was granted this privilege, and the dual high command structure then in place caused problems in command and control. It also contributed to the Union's defeat at the Battle of the Second Bull Run.
General Tecumseh Sherman, was the Union General who ransacked the South, all the way to the sea. He took over the Union Campaign in the west after General Ulysses S. Grant took over command of the Union Army, or the Army of the Potomac.
The Union field army involved was the Army of the Potomac, at that time under the command of Major General Joseph Hooker. Under "related links" below is a link to a Union "Order of Battle" for the Army of the Potomac at Chancellorsville which lists all units in a particular command at a certain moment in time.
At the end of July, 1861, Union General George B. McClellan was in charge of the Division of the Potomac. Under McClellan, was General K. F. Mansfield, in command of the District of Columbia and General McDowell in command of a division.
General Meade, still in command of the Army of the Potomac was forced to send two corps to Tennessee to help reinforce General Rosencrans.
General Robert E. Lee commanded the Confederate Army of Northern Virginia. General George B. McClellan commanded the Union Army of the Potomac.
The Union had several field armies, including the Army of the Potomac. After McClellan it was commanded by Ambrose Burnside, Joseph Hooker, George Meade. When Grant came east he was made general in chief over all Union armies, but made his HQ with the Army of the Potomac and traveled with it. Meade remained in command of the Army of the Potomac, but had Grant right with him, supervising.