Probably the Union's most able Cavalry leader. He was in charge of the Cavalry of the Army of the Potomac under Grant. Sheridan had a fierce temper, and it nearly cost him his career. At West Point he got into a fight with fellow-cadet (and fellow future-general) William Terrill. He wasn't expelled, but he had to serve an extra year before graduating in 1853. He went to the infantry and had the normal ' i.e. slow ' promotions.
He was a First Lieutenant in 1861 and was jumped only as far as Captain. That took him onto the staff, as a quartermaster and commissary officer in southwest Missouri. His potential was spotted by Gordon Granger, possibly Granger's greatest service to the Union. Sheridan was given command of the 2nd Michigan Cavalry (May 1862) and within a week was commanding a cavalry brigade. By the end of September 1862 it was an infantry division. He led them at Perryville and Murfreesboro, then at Chickamauga. He couldn't prevent his troops rout there, but later in the year brightened his reputation by breaking through the impregnable' Confederate line on Missionary Ridge at Chattanooga. He probably didn't expect his men could achieve so much and he certainly didn't exploit far. When Grant was promoted to general-in-chief he sent for Sheridan who took over the cavalry of the Army of the Potomac.