Mac had wanted more men for the offensive, but Lincoln wanted him to hold men back to guard D.C. Lincoln forced Mac to leave McDowell’s I Corps in D.C. along with the garrison already available. Unlike Lincoln, Mac saw Jackson's valley campaign as a diversion by Lee (now in command of the Southern Army) to pull federals away to help protect Richmond. On Sep 16th, Mac trapped Lee by a flank maneuver on Lee's left and captured the road to Hagerstown, forcing Lee's northern invasion to end.