As darkness fell, the rest of D.H. Hill's division came up followed by Longstreet, while on the Union side, George Morrell's division arrived and relieved McCall, whose men were nearly out of ammunition. There was not enough daylight remaining to deploy D.H. Hill and Longstreet's divisions. Jackson did not attack, but his presence near Porter's flank caused McClellan to order Porter to withdraw after dark behind Boatswain's Swamp, five miles (8 km) to the east. McClellan was concerned that the Confederate buildup on his right flank threatened his supply line, the Richmond and York River Railroad north of the Chickahominy, and he decided to shift his base of supply to the James River (he also believed that the demonstrations by Huger and Magruder showed that he was seriously outnumbered). This was a strategic decision of grave import because it meant that, without the railroad to supply his army, he had to abandon his siege of Richmond.