At the Battle of Chancellorsville, Howard suffered the first of two significant military setbacks. On May 2, 1863, his corps was on the right flank of the Union line, northwest of the crossroads of Chancellorsville. Robert E. Lee and Lt. Gen. Thomas J. "Stonewall" Jackson devised an audacious plan in which Jackson's entire corps would march secretly around the Union flank and attack it. Howard was warned by Maj. Gen. Joseph Hooker, now commanding the Army of the Potomac, that his flank was "in the air", not anchored by a natural obstacle, such as a river, and that Confederate forces might be on the move in his direction. Howard failed to heed the warning and Jackson struck before dark, routing the XI Corps and causing a serious disruption to the Union plans.