As the battle shifted away from the Cornfield, General John Sedgwick's division of Federal troops, personally led by corps commander General Edwin Sumner, advanced from the East Woods with a plan to move into the West Woods, strike the Confederates in the flank, and drive them toward Sharpsburg. At 9:15 a.m., more than 5,000 Union soldiers marched out of the East Woods toward the West Woods. At the same time, among other Confederate forces, General Lafayette McLaws's division entered the fight to shore up the left end of the Confederate line, and as Confederates pressed in from one side and Sedgwick's men from the other, the convergence proved disastrous for the Union troops. Confederate attacks struck from three sides into the flanks of the Federal line, the fighting became so confused that New York men fired into the backs of soldiers in front of them, and within twenty minutes the Union troops fell back from the West Woods toward the north and east. As Sedgwick's men fled, the other two divisions of the Second Corps assaulted the Sunken Road. Sedgwick lost 2,200 of 5,300 men in twenty minutes, and the 15th Massachusetts suffered 340 casualties, the highest number for any regiment on the field during the battle. By 11:00 a.m., other Union soldiers held part of the West Woods around the Dunker Church, but within an hour they too were driven back to the East Woods, and Federal forces did not venture west of the Hagerstown Turnpike again that day. Early that morning, General William E. Starke took command of the Stonewall Division after General John R. Jones was stunned by an artillery shell and left the field; as the Union First Corps began to drive his men back, Starke led a counterattack out of the West Woods, was wounded three times, and died within the hour, after which his body was returned to Richmond for burial in Hollywood Cemetery. In their first battle, the 125th Pennsylvania Infantry held the left flank of the Union advance into the West Woods and took the brunt of the Confederate attack; color bearer Sergeant George Simpson was killed instantly, but the flag was saved and forty-two years later was brought back for the dedication of the regiment's monument behind the Dunker Church. The West Woods Massacre unfolded from about 9:30 a.m. to 10:30 a.m., with about 6,000 Union and 8,000 Confederate soldiers engaged, and about 2,300 casualties in the Union Army of the Potomac and 2,000 in the Confederate Army of Northern Virginia.
