George B. McClellan entered the army as an engineer after graduating near the top of his West Point class in 1846. He served in the Mexican-American War, taught at the academy, surveyed railroad routes, and studied European armies during the Crimean War. He left military service in 1857 for railroad management, bringing engineering and organizational experience back to the army when the Civil War began.

Early operations in western Virginia in 1861 made his reputation, and after the defeat at First Bull Run he was called to Washington. He organized and trained the Army of the Potomac, winning strong affection among its soldiers. For several months he also served as general in chief. His caution, recurring overestimates of Confederate strength, and difficult relationship with Abraham Lincoln became central problems as the administration pressed for decisive operations.

In the Peninsula Campaign of 1862 McClellan moved his army by water and advanced toward Richmond. The campaign ended without taking the Confederate capital after the fighting of the Seven Days. Recalled to command around Washington after another Union defeat, he confronted Robert E. Lee's invasion of Maryland. Fighting at South Mountain and Antietam forced Lee to abandon that invasion, but McClellan did not destroy the Confederate army or pursue it vigorously. Lincoln removed him from field command in November 1862. His earlier removal as general in chief had occurred that March.

Democrats nominated McClellan for president in 1864. Although the party platform called the war a failure, he insisted on restoration of the Union and rejected peace through permanent separation. Lincoln defeated him decisively. McClellan received no further military command, but resumed engineering and business work and served as governor of New Jersey from 1878 to 1881. His skill in creating an army exceeded his effectiveness in directing it, while his politics placed him against Lincoln.