When Vicksburg fell, Washington promoted Grant to major general in the U.S. Army, which meant that Grant, already a major general of volunteers, would retain his rank after the war. Only three other officers on active duty held this rank, none of them commanding in the field.   At Vicksburg Grant supervised the parole of 30,000 pris­oners. His victory there had opened trade on the Mississippi; for a short time his duty also consisted of making sure the Union, not the Confederacy, benefited from this newly opened route.   At the end of August Grant went to New Orleans to confer about an attack on Mobile, Alabama. After being sidelined fol­lowing a fall from his horse-- which revived rumors of his drinking-- Grant entered Chattanooga to open a supply line to besieged Chickamauga, Georgia. He then coordinated an assault that delivered Chickamauga into Union hands, and before the end of the year he had driven the Confederates from Tennessee.   Congress voted him a gold medal, discussed a bill to revive the rank of lieutenant general, and both parties considered him as a potential candidate for Congress. Grant carefully composed his letters to discourage his political supporters. As usual, Grant meant what he said: he was a soldier who wanted the oppor­tunity to fulfill his responsibility.