Daniel Sickles was a Major General in the Army of the Potomac and was in command of Union III corps at Gettysburg. III corps is made up of 6 brigades split into 2 divisions.
Gettysburg was to mark the effective end, of Sickles's military career. Meade had ordered Sickles's corps to take up defensive positions on the southern end of Cemetery Ridge, anchored in the north to the II Corps and to the south, the hill known as Little Round Top.
Sickles was unhappy to see a slightly higher terrain feature to his front, the Peach Orchard. Perhaps remembering the beating his corps took from Confederate artillery at Hazel Grove, he violated his orders and marched his corps almost a mile in front of Cemetery Ridge. This had two effects: it greatly diluted the concentrated defensive posture of his corps, by stretching it too thin; and it created a salient that could be bombarded and attacked from multiple sides. Meade rode out and confronted Sickles about his insubordination, but it was too late. The Confederate assault by Longstreet’s corps, smashed III Corps and rendered it useless for further combat.
During the height of the Confederate attack, Sickles fell victim to a cannonball that mangled his right leg. Carried by stretcher to an aid station, he bravely attempted to raise his soldiers' spirits by grinning and puffing on a cigar along the way. His leg was amputated that afternoon