The battle opened at dawn on September 17, 1862 with an attack down the Hagerstown Turnpike by the Union I Corps under Joseph Hooker. Hooker's objective was the plateau on which sat the Dunker Church, a modest whitewashed building belonging to a local sect of German Baptists. Hooker had approximately 8,600 men, little more than the 7,700 defenders under Stonewall Jackson, and this slight disparity was more than offset by the Confederates' strong defensive positions. Abner Doubleday's division moved on Hooker's right, James Ricketts's moved on the left into the East Woods, and George Meade's Pennsylvania Reserves division deployed in the center and slightly to the rear. Jackson's defense consisted of the divisions under Alexander Lawton and John R. Jones in line from the West Woods, across the Turnpike, and along the southern end of the Miller Cornfield. Four brigades were held in reserve inside the West Woods. As the first Union men emerged from the North Woods and into the Cornfield, an artillery duel erupted. Confederate fire was from the horse artillery batteries under Jeb Stuart to the west and four batteries under Col. Stephen D. Lee on the high ground across the pike from the Dunker Church to the south. Union return fire was from nine batteries on the ridge behind the North Woods and twenty 20-pounder Parrott rifles, 2 miles (3 km) east of Antietam Creek. The conflagration caused heavy casualties on both sides and was described by Col. Lee as "artillery Hell". Seeing the glint of Confederate bayonets concealed in the Cornfield, Hooker halted his infantry and brought up four batteries of artillery, which fired shell and canister over the heads of the Federal infantry, covering the field. All at once, the cornfield exploded into chaos as a savage battle raged through the area. Men beat each other over the heads with rifle butts and stabbed each other with bayonets. Officers rode around on their horses sweating and cursing and yelling orders no one could hear in the noise. Rifles became hot and fouled from too much firing. The air was filled with a hail of bullets and shells. The morning phase ended with casualties on both sides of almost 13,000, including two Union corps commanders.