The Irish 9th Regiment Massachusetts Volunteer Infantry regiment, part of the Army of the Potomac of the Union Army, is heavily engaged at the Battle of Gaines’ Mill, sometimes known as the Battle of Chickahominy River, in Hanover County, Virginia, on June 27, 1862. It is the third of the Seven Days Battles (Peninsula campaign) of the American Civil War.

Following the inconclusive Battle of Beaver Dam Creek (Mechanicsville) the previous day, Confederate General Robert E. Lee renews his attacks against the right flank of the Union Army, relatively isolated on the northern side of the Chickahominy River. There, Brigadier General Fitz John Porter‘s V Corps establishes a strong defensive line behind Boatswain’s Swamp. Lee’s force is destined to launch the largest Confederate attack of the war, about 57,000 men in six divisions.

Porter’s reinforced V Corps holds fast for the afternoon as the Confederates attack in a disjointed manner, first with the division of Major General A. P. Hill, then Major General Richard S. Ewell, suffering heavy casualties. Put into an exposed, forward position near the bridge over Powhite Creek, the 9th Massachusetts sustains heavy casualties while delaying the advance of A.P. Hill’s division, allowing other Federal forces to improve their defenses. Among the Confederates attacking the 9th’s position are the Irishmen of Company K, 1st South Carolina Rifle Regiment.

After pulling back to the main Federal line, the 9th Massachusetts regiment is hotly engaged again later in the day. Numerous attacks by Hill’s Confederates are repulsed through the day, and the 9th also helps cover the retreat of their brigade. The 9th Massachusetts is one of the last regiments of the V Corps remaining on the field as General Thomas Francis Meagher and his Irish Brigade rush into line to relieve the beleaguered remnant of the brave Massachusetts regiment. The arrival of Major General Stonewall Jackson‘s command is delayed, preventing the full concentration of Confederate force before Porter receives some reinforcements from the VI Corps.

Seeing the green flags of the Irish Brigade coming to the aid of the 9th Massachusetts, Lieutenant Colonel Patrick Guiney, who had been watching his regiment shrink in number all day, shakes the hand of Meagher and exclaims, “Thank God, we are saved.”

At dusk, the Confederates finally mount a coordinated assault that breaks Porter’s line and drives his men back toward the Chickahominy River. The Federals retreat across the river during the night. The Confederates are too disorganized to pursue the main Union force. Gaines’ Mill saves Richmond for the Confederacy in 1862. The tactical defeat there convinces Army of the Potomac commander Major General George B. McClellan to abandon his advance on Richmond and begin a retreat to the James River. The 9th Massachusetts’ loses for the day are 82 killed and 167 wounded.

The battle occurs in almost the same location as the Battle of Cold Harbor nearly two years later.