McClellan’s plan called for an initial attack on the Confederate left flank on the Hagerstown Pike with the two corps of Hooker and Mansfield. McClellan intended to support this mass charge with Sumner’s entire force and, if necessary, with Franklin’s corps. If the powerful thrust against the Confederate left should succeed, McClellan would send Burnside’s corps across Antietam Creek at the Lower Bridge and strike the Confederate right flank on the ridge southeast of Sharpsburg. Should Burnside succeed in turning the southern end of Lee’s line, he would be expected to carry the attack northwest toward Sharpsburg. Finally, if either of these flanking movements appeared successful, McClellan would drive up the Boonsboro Pike with all available forces to smash the Confederate center. [Illustration: _Meadow just beyond trees bordering Antietam Creek marks top of bluffs where many of Hunt’s Union batteries were placed. This view from one-half mile in front of Confederate gun emplacements on Sharpsburg Ridge._] It was a good plan. If the Federal attacks could be delivered in concert, McClellan’s preponderance of power must stretch Lee’s smaller force to the breaking point. But the story of Antietam is one of piecemeal Federal attacks—a corps here, a division there. This failure in execution allowed Lee to shift troops from momentarily quiet sectors to plug the gaps torn by the succession of Federal attacks. As each threat developed, Lee rushed his troops there and beat it back. Taking advantage of his interior lines, he repeatedly achieved a local advantage of numbers, though larger Federal contingents were always nearby. [Illustration: _Brig. Gen. W. N. Pendleton, Lee’s chief of artillery._ From Miller’s Photographic History of the Civil War.] [Illustration: _Brig. Gen. Henry Hunt, McClellan’s chief of artillery._] _The Lines Are Poised for Action_ At 2 p.m. on the 16th, Hooker marched from his camp near Keedysville, crossed the Upper Bridge, and late in the afternoon reached the Hagerstown Pike. Under cover of the North Woods, his divisions formed for the attack on both sides of the pike. A massed force of more than 12,000 men was ready to advance on the Confederates. [Illustration: _Union artillery in battery line._ From 1863 photograph. Courtesy, Library of Congress.] Lee’s thin line, 3 miles long, had been reinforced early on the 16th by the arrival of Jackson’s troops from Harpers Ferry. They were placed where they could support the northern part of the Confederate line. John Walker’s division, arriving from Harpers Ferry in the afternoon, took position south of Sharpsburg. Jackson now commanded the Confederate front north of Sharpsburg; Longstreet, with a part of his force north of the village, extended the line nearly a mile south.