The major crisis facing Johnston was defending Confederate control of Vicksburg, Mississippi, which was threatened by Union Maj. Gen. Ulysses S. Grant, first in a series of unsuccessful maneuvers during the winter of 1862–63 to the north of the fortress city, but followed in April 1863 with an ambitious campaign that began with Grant's Union army crossing the Mississippi River southwest of Vicksburg. Catching Lt. Gen. Pemberton by surprise, the Union army waged a series of successful battles as it moved northeast toward the state capital of Jackson. On May 9, the Confederate Secretary of War directed Johnston to "proceed at once to Mississippi and take chief command of the forces in the field." Johnston informed Richmond that he was still medically unfit, but would obey the order. When he arrived in Jackson on May 13 from Middle Tennessee, he learned that two Union army corps were advancing on the city and that there were only about 6,000 troops available to defend it. Johnston ordered a fighting evacuation (the Battle of Jackson, May 14) and retreated with his force to the north. Grant captured the city and then faced to the west to approach Vicksburg.