On May 14, the Union army attacked Jackson. Johnston withdrew his men from the city, with Gregg performing rear guard duty. Grant's army then turned west and encountered Pemberton's men, who were attempting to make a stand east of Vicksburg. On May 16, Pemberton' men were defeated at the Battle of Champion Hill, and after a defeat in a rear guard action at the Battle of Big Black River Bridge the next day, withdrew into the Vickburg defenses. Union attempts to take the city by direct assault on May 19 and 22 failed, and Grant placed Vicksburg under siege. Supplies within the city eventually ran low, and with no hope of escape, Pemberton surrendered the city and his army on July 4, ending the Siege of Vicksburg. The capture of Vicksburg was a critical point in the war.