On May 22, 1863, Ulysses Grant sent brigades from three corps of the army to assault Vicksburg. While the assault showed some success, a long bitter struggle ensued and the Confederates quickly restored their original lines of defense. Realizing that the city could not be taken by assault, Grant ordered his engineers to begin siege operations. The siege cut off all supplies going into the city and the constant hammering of siege artillery drove many of the citizens into caves dug into the hillsides.