The Federals were completely unaware that 3,000 new men would be facing them. Burnside's plan was to move around the weakened Confederate right flank, converge on Sharpsburg, and cut Lee's army off from Boteler's Ford, their only escape route across the Potomac. At 3 p.m., Burnside left Sturgis's division in reserve on the west bank and moved west with over 8,000 troops (most of them fresh) and 22 guns for close support.^[38]

An initial assault led by the 79th New York "Cameron Highlanders" succeeded against Jones's outnumbered division, which was pushed back past Cemetery Hill and to within 200 yards (200 m) of Sharpsburg. Farther to the Union left, Rodman's division advanced toward Harpers Ferry Road. Its lead brigade, under Col. Harrison Fairchild, containing several colorful Zouaves of the 9th New York, commanded by Col. Rush Hawkins, came under heavy shellfire from a dozen enemy guns mounted on a ridge to their front, but they kept pushing forward. There was panic in the streets of Sharpsburg, clogged with retreating Confederates. Of the five brigades in Jones's division, only Toombs's brigade was still intact, but he had only 700 men.^[39]