During the Chickamauga Campaign, General David Stanley's two Union cavalry divisions battled Nathan Bedford Forrest's and Joseph Wheeler's two cavalry corps in some of the most difficult terrain for mounted operations in the Civil War. The Federal cavalry divisions, commanded by George Crook and Edward McCook, secured the flanks on the Union advance on Chattanooga, secured the crossing of the Tennessee River, and then pushed into enemy-held territory. Cavalry fights at Alpine and La Fayette marked the early part of the campaign, but the battle exploded on September 18 as Col. Robert Minty and Col. John Wilder held back a determined attack by Confederate infantry, reminiscent of Buford's actions at Gettysburg. Due to Stanley's illness, Robert Mitchell assumed command of the cavalry during the battle along Chickamauga Creek, with notable cavalry actions at Glass Mill, Cooper's Gap, and securing the flanks after the battle. Soon thereafter, the Union cavalry fought Wheeler's mounted forces raiding through Tennessee before the battle at Farmington sent the Confederate horsemen back across the Tennessee River. The contributions of the Union cavalry during this campaign are often overlooked, but the troopers fought through conditions so dusty they could hardly see the horse in front of them while boldly leading the infantry in the second costliest battle in the Civil War.