General George B. McClellan was a prominent Union general during the American Civil War. Born on December 3, 1826, he graduated from the United States Military Academy and served in the Mexican-American War. McClellan is best known for his role as the commander of the Army of the Potomac, where he organized and trained troops, significantly improving their readiness for battle. Despite his organizational skills, McClellan faced criticism for his cautious approach in combat. He led the Union forces during the Peninsula Campaign in 1862 but was ultimately replaced by President Abraham Lincoln due to his reluctance to engage decisively with Confederate forces. McClellan later ran for president in 1864 but lost to Lincoln.