Which Union general was known as 'the Rock of Chickamauga'?

The Union general known as “the Rock of Chickamauga” was George Henry Thomas.

Thomas commanded the Union XIV Corps during the Battle of Chickamauga in Georgia on September 19–20, 1863. When much of the Union Army of the Cumberland collapsed under Confederate pressure, Thomas held his position on Horseshoe Ridge and organized a stubborn defense. His stand prevented the retreat from becoming a complete rout.

The nickname grew from a message associated with Major General James A. Garfield, then serving on William Rosecrans’s staff. Garfield reported that “Thomas is standing like a rock,” a phrase that soon became the general’s most famous sobriquet. Although Chickamauga ended in a Confederate tactical victory, Thomas’s defense preserved the Union army and allowed it to withdraw toward Chattanooga.

Thomas was a Virginian who remained loyal to the United States when Virginia joined the Confederacy. He later commanded the Army of the Cumberland and won the decisive Union victory at Nashville in December 1864. “Rock of Chickamauga” specifically refers to his defensive stand there, not to his later Nashville victory.