on may 9, gen. johnston received a dispatch from the confederate secretary of war directing him to "proceed at once to mississippi and take chief command of the forces in the field." as he arrived in jackson on may 13, from middle tennessee, he learned that two army corps from the union army of the tennessee—the xv, under maj. gen. william tecumseh sherman, and the xvii, under maj. gen. james b. mcpherson—were advancing on jackson, intending to cut the city and the railroads off from vicksburg, mississippi which was a major port on the mississippi river. these corps, under the overall command of grant, had crossed the mississippi river south of vicksburg and driven northeast toward jackson. the railroad connections were to be cut to isolate the vicksburg garrison. and if the confederate troops in jackson were defeated, they would be unable to threaten grant's flank or rear during his eventual assault on vicksburg.