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Gaines’s Mill was the third battle during the Seven Days’ Battles that ended General McClellan’s Peninsula campaign of 1862. At the start of the Seven Days, McClellan’s army had been split in two by the Chickahominy River. One corps, under Fitz-John Porter, was north of the river, and the rest of the army was on the south bank. Porter’s corps was protecting McClellan’s supply lines, back to Whitehouse.
Robert E. Lee saw the opportunity this arrangement gave him. He was expected the arrival of Stonewall Jackson and his army from the Shenandoah Valley. Once that army arrived, Lee planned to combine it with as many of his own men as possible and smash Porter’s corps. He hoped to defeat McClellan’s larger army section by section.
The battle did not go as planned. Jackson’s men had a reputation for rapid movement, but by now they were exhausted, and arrived two days later than expected. This meant that Lee’s men fought unsupported at Mechanicsville (26 June) and suffered a serious defeat. However, McClellan turned this defeat into a victory by deciding to abandon his supply lines north, and instead move to the James River, where he could be supplied by sea. Overnight on 26-27 June, Porter’s corps was moved back to Gaines’s Mill, much nearer to the bridges over the Chickahominy.
Jackson’s men had now arrived outside Richmond. Lee’s plan for 27 June had A. P. Hill attack Porter’s centre, Longstreet feint against his left and Jackson attack his right. If Porter weakened his left, then Longstreet would launch proper attack. Delivered well, this would see 55,000 Confederate troops attack 35,000 Union men.
A key role in this plan was to be played by General Magruder. Having delayed McClellan on the Peninsula, he was now to repeat his theatricals south of the Chickahominy. He had 27,000 men, and was outnumbered nearly three to one by McClellan’s 69,000 south of the Chickahominy. Showing the same skills as on the Peninsula, aided by McClellan’s willingness to believe his enemy had a vast army, Magruder was able to pin McClellan in place.
Once again Jackson did not do well. While A. P. Hill went in as planned, Jackson’s attack was late and badly coordinated. For several hours Hill’s men fought almost alone, taking heavy casualties. Some relief was provided by Longstreet and by those of Jackson’s men who did get into the action.
Finally, late in the day, Lee was able to organise a proper coordinated attack along his entire line. Porter’s line was finally broken by John Hood’s Texans, and came close to routing. Now McClellan finally moved some men north across the Chickahominy to form a rearguard, allowing Porter to get the bulk of his men safely across to the south bank. Gaines’s Mill was the only clear Confederate victory during the Seven Days’ Battles. Even so, it was won at heavy cost. Union losses of 6,800 were inflicted at a cost of 9,000 Confederate casualties.