Henry Slocum commanded XII Corps, Army of the Potomac, during the Battle
of Gettysburg. Upon his arrival on the afternoon of 1 July, Slocum, knowing
that he would be given overall command of the field upon reporting, was
reluctant to go to Cemetery Hill to confer with the other corps commanders.
Eventually, realizing that there were no alternatives, he went and Winfield
S. Hancock turned over to him the control of all Federal forces then engaged.
Slocum positioned his troops on Culp's Hill to the right of those from Solomon Meredith's Iron Brigade of James Wadsworth's 1st Division, I Corps, who had been ordered to Culp's Hill by Hancock. On 2 July, George Meade, fearing that the assault on his left might succeed, ordered Slocum's corps to move to support Daniel Sickles' and George Sykes' corps. Slocum manged to persuade Meade to leave one of his brigades, that of George Greene's New Yorkers--60, 78, 102, 137, 149 NY--on Culp's Hill, and this brigade, especially John Ireland's 137th NY, saw heavy action that evening and the following day.
On 3 July, at about 4:30 AM, troops from XII Corps made a preëmptive strike against the Confederate troops occupying the area around Spangler's Spring while they were planning their own attack at about 7:00 AM to coincide with what they thought would be Jame Longstreet's assault on the Federal left. The right of the Federal lines on Culp's Hill settled into some heavy skirmishing while the left and center received and repulsed three assaults by Edward Johnson's division of Richard Ewell's Corps.
Photo from the National Archives.
From Peter Schwartz' HomePage