He was upset because they had just won the 1st real, (Antietam was claimed as a victory, but was really a draw), undoubted victory, but instead of following Lee & mercilessly doing every thing that he could to destroy the Army of Virginia while it was far from home, he let them go without any real pursuit. It lengthened the war by years, it is very possible that had Meade been more ruthless, the war could have been over in months, instead of the 2 more years it took.
Major General George G. Meade was in command of the Union forces at the Battle of Gettysburg.
President Lincoln was frustrated with George Meade because of his failure to pursue Lee's army after the Battle of Gettysburg. This move could have possibly ended the war had Lee's army been completely destroyed. By allowing him to escape, Meade also allowed the fighting to continue.
Meade
MEADE
George Gordon Meade fought in The Battle of Gettysburg.
Meade commanded the Army of the Potomac at Gettysburg.
The Union did: Grant at Vicksburg, Meade at Gettysburg.
George Meade commanded the Union forces at Gettysburg.
Because Meade could have pursued and destroyed Lee's army. Lincoln criticised him heavily for failing in this.
One of the wars that Gorge meade was in was the Battle of Gettysburg
George Meade
George Gordon Meade