McClellan left West Point in 1851 for a series of engineering assignments in Delaware, Arkansas, Texas, and the Cascade Mountains in the Pacific Northwest.  In 1855, he joined a board of officers for a year abroad studying military systems in Europe and the Crimean War Theater, for which he wrote highly regarded reports.  Based on his observations, he made several suggestions for improving the American armed forces, including a new kind of saddle.  In 1857, he retired from the army to work in the railroad industry.  He was chief engineer, then vice president, of the Illinois Central Railroad before accepting the presidency of the Ohio and Mississippi Railroad in 1860. 

When the Civil War began, McClellan was named major general in charge of the Ohio volunteers and state militia, but within a month was appointed major general in the federal army and put in charge of the Department of Ohio.  His victory at Rich Mountain, West Virginia (July 11, 1861), just ten days before the Union defeat at Bull Run, brought him to the attention of the military’s top echelon.  As a result, he was given command of the Division of the Potomac, centered around Washington, D. C.  Finding the troops in disarray, he reorganized, trained, and disciplined them, who affectionately nicknamed him the “Young Napoleon.”  He was a reluctant fighter, though, who continually overestimated enemy troop strength and hesitated to put his troops into action. 

On August 15, 1861, McClellan assumed command of the Army of the Potomac, the major Union force in the Eastern Theater, and in November 1861, replaced General Winfield Scott, who retired, as general-in-chief.  The appointment prompted McClellan to delay military action further as he pondered the larger military situation he had inherited.  Lincoln's exasperation with his top general's slowness to battle, as well as disputes over strategy, led the president to relieve McClellan of the post of general-in-chief on March 11, 1862, although leaving him in charge of the Army of the Potomac.  

After Confederate troops pushed Union forces away from the Confederate capital of Richmond in the Seven Days Campaign (June 24-July 1, 1862) and defeated the Union at the Second Battle of Bull Run (August 28-30, 1862), General Robert E. Lee invaded the Union state of Maryland.  Lee hoped that his presence would inspire Confederate sympathizers in the state, and that a major victory would strengthen the Union peace movement and undermine Union morale.  As Lee moved into western part of the state, McClellan had the good fortune to be given a mislaid copy of Lee's plans, which indicated that Confederate troops were currently scattered.  However, instead of seizing the initiative, McClellan waited, allowing some of the Confederates time to reassemble at Antietam Creek near the town of Sharpsburg, Maryland.

The Battle of Antietam on September 6-7, 1862, was the bloodiest of the entire war.  McClellan with 87,000 men repeatedly assaulted Lee, whose 50,000 men were reinforced by Confederate troops under Stonewall Jackson.  At one point in the battle, when some Union troops found themselves trapped, 2,000 men were killed in a matter of minutes.  In all, nearly 23,000 men were killed, wounded, or missing--13,000 Union soldiers and 10,000 Confederates.  The casualties at Antietam numbered four times the total suffered by American soldiers on the beaches of Normandy during World War II, and more than twice as many Americans lost their lives in one day of the battle as died in combat during the War of 1812, the Mexican-American War, and the Spanish-American War combined.

McClellan never saw field duty again, but in 1864, the Democratic Party nominated him for president. The party platform reflected the dominant force of the Peace Democrats ("Copperheads") at the national convention.  It criticized Lincoln’s administration of the war effort, the suppression of civil liberties, and called for an immediate cessation of fighting and a negotiated settlement.  McClellan repudiated the “peace plank,” promising, instead, to prosecute the war more effectively than Lincoln.  The president defeated McClellan by a large margin in the Electoral College.