Crook led his brigade at the Battle of South Mountain and near Burnside's Bridge at the Battle of Antietam. He was promoted to the rank of brigadier general on September 7, 1862. During these early battles he developed a lifelong friendship with one of his subordinates, Col. Rutherford B. Hayes of the 23rd Ohio Infantry. Following Antietam, General Crook assumed command of the Kanawha Division. His division was detached from the IX Corps for duty in the Department of the Ohio. Before long Crook was assigned to command an infantry brigade in the Army of the Cumberland. This brigade became the 3rd Brigade, 4th Division, XIV Corps, which he led at the Battle of Hoover's Gap. In July he assumed command of the 2nd Division, Cavalry Corps in the Army of the Cumberland. He fought at the battle of Chickamauga and was in pursuit of Joseph Wheeler during the Chattanooga Campaign. In February 1864, Crook returned to command the Kanawha Division, which was now officially designated the 3rd Division of the Department of West Virginia. To open the spring campaign of 1864, Lieutenant General Ulysses S. Grant ordered a Union advance on all fronts, minor as well as major. Grant sent for Brigadier General Crook, in winter quarters at Charleston, West Virginia, and ordered him to attack the Virginia and Tennessee Railroad, Richmond's primary link to Knoxville and the southwest, and to destroy the Confederate salt works at Saltville, Virginia. The 35-year-old Crook reported to army headquarters at City Point, Virginia, where the commanding general explained the mission in person. Grant instructed Crook to march his force, the Kanawha Division, against the railroad at Dublin, Virginia, south of Charleston. At Dublin he would put the railroad out of business and destroy Confederate military property. He was then to destroy the railroad bridge over New River, a few miles to the east. When these actions were accomplished, along with the destruction of the salt works, Crook was to march east and join forces with Major General Franz Sigel, who meanwhile was to be driving south up the Shenandoah Valley. After long dreary months of garrison duty, the men were ready for action. Crook did not reveal the nature or objective of their mission, but everyone sensed that something important was brewing. "All things point to early action", the commander of the second brigade, Colonel Rutherford B. Hayes, noted in his diary. On April 29, 1864, the Kanawha Division marched out of Charleston and headed south.