On July 26, 1861, Union General George McClellan took command from McDowell of the Army of the Potomac. An excerpt from the article:
"George Brinton McClellan is often remembered as the great organizer of the Union Army of the Potomac. Nicknamed "Young Napoleon," "Little Mac" was immensely popular with the men who served under his command. His military command style, however, put him at odds with President Abraham Lincoln, and would ultimately upset his military and political fortunes.
McClellan began his military career after entering the United States Military Academy in 1842. He graduated second in a class of 59 in 1846, along with 20 others who would become full rank generals during the Civil War. He was appointed as a brevet second lieutenant in the Corps of Engineers and served under General Winfield Scott during the Mexican-American War, helping to construct roads and bridges for the army. The recipient of brevet promotions to both first lieutenant and captain, he returned to West Point as an instructor after the war, and helped translate a French manual on bayonet tactics. Other duties included service as an engineer at Fort Delaware, expeditions to explore the Red River, and the exploration of possible routes for the transcontinental railroad. He was also a military observer during the Crimean War. In 1857, McClellan resigned from the military to take a position with the Illinois Central Railroad.
Upon the outbreak of the Civil War, Ohio governor William Dennison appointed McClellan major general of Ohio Volunteers on April 23, 1861. This promotion, along with the support of Governor Dennison, encouraged Lincoln to commission McClellan a major general in the Regular Army, making him one of the highest ranked individuals in the service, under only Winfield Scott. McClellan began his work swiftly, ensuring that Kentucky would not secede from the Union. He then commanded forces during the Rich Mountain campaign in what is now West Virginia, to ensure that the portion of the state would not be fully taken by Confederates. This success, combined with the defeat of General Irvin McDowell at the Battle of First Bull Run, led McClellan to become Commander of the Army of the Potomac, and later General-in-Chief of all Federal armies upon the retirement of General Winfield Scott in November 1861."