The second battle during the Seven Days’ Battles that ended the Peninsula campaign. After the Battle of Seven Pines, Robert E. Lee had been appointed to command the Confederate armies around Richmond. He decided that the only change of preventing George McClellan’s large Union army from capturing the Confederate capitol was to attack while that army was split in two by the Chickahominy River. He hoped to combine the army already around Richmond with Stonewall Jackson’s army from the Shenandoah Valley. If successful, Lee and Jackson would be able to defeat the larger Union army in detail.
Meanwhile McClellan was finally preparing to launch his attack on Richmond. On 25 June he had ordered a minor advance on his left wing (south of the Chickahominy), in preparation for a planned attack further north on 26 June. However, by now McClellan was aware that Jackson’s men were on their way, and the attack was cancelled.
McClellan’s army was hamstrung by two problems. The first was that it was divided by the Chickahominy River. One entire corps (Porter’s) was north of the river, guarding the routes north and east while the rest of the army was on the south bank, directly threatening Richmond. The second problem was that McClellan was convinced that he was outnumbered, estimating Confederate strength as close to 200,000, twice its real size.
Lee’s plan was to launch an attack with Jackson’s troops from the valley and three divisions from around Richmond (Longstreet’s, A. P. Hill’s and D. H. Hill’s). This plan had one serious flaw. It relied on the prompt arrival of Jackson’s army. That army had just fought and won an exhausting campaign in the Shenandoah Valley, and was finally starting to show the strain. Jackson had originally predicted that he would arrive on 25 June, but had failed. He was now expected early on 26 June. He was to attack Porter’s right flank, and his attack would be the signal for Lee’s men to launch their own attack against Porter’s front.
Unfortunately, Jackson simply didn’t arrive. He and his men simply could not keep up the pace they had set for so long in the valley. Eventually, in mid-afternoon, A. P. Hill could wait no longer and launched his attack. As one would expect, this unsupported attack against entrenched opponents was a disastrous failure. The Confederates lost 1,500 men against Union losses of only 360.
Although Mechanicsville was a clear Confederate defeat, McClellan proceeded to turn it into the beginnings of a victory. Worried about his supply lines, McClellan decided to chance his base, from Whitehouse, north of the Chickahominy River, to the James River. Ironically Jackson’s failure to appear probably played a part in that decision – if Lee was strong enough to attack without Jackson, what might he achieve once his entire army was present! Accordingly, on the night of 26-27 June, Porter moved back to Gaines’s Mill, nearer to the bridges over the Chickahominy. There, they would fight another battle on the following day.