MacClellan, George Brinton (1826-85), an American general, was born at Philadelphia, and trained at the West Point military school. He served in the Mexican War and the Red River expedition, and on the outbreak of civil war succeeded McDowell after the battle of Bull's Run, and became commander-in-chief (1861). He then commanded the army of the Potomac. Later he compelled Lee to retire from Maryland by the battles of South Mountain and Antietam (September 14-17, 1862); but owing to his apparent indolence he was relieved of his command and retired from the army. In 1864 he was the Democratic nominee for the presidency, but was defeated by Abraham Lincoln. In later life he was superintendent of a railway.