 George Brinton McClellan ( December 3 , 1826 – October 29 , 1885 ) was an American soldier , civil engineer , railroad executive , and politician . A graduate of West Point , McClellan served with distinction during the Mexican @-@ American War , and later left the Army to work in railroads until the outbreak of the American Civil War . Early in the war , McClellan was appointed to the rank of major general and played an important role in raising a well @-@ trained and organized army , which would become the Army of the Potomac ; he served a brief period ( November 1861 to March 1862 ) as general @-@ in @-@ chief of the Union Army . Although McClellan was meticulous in his planning and preparations , these very characteristics hampered his ability to challenge aggressive opponents in a fast @-@ moving battlefield environment . He chronically overestimated the strength of enemy units and was reluctant to apply principles of mass , frequently leaving large portions of his army unengaged at decisive points . 
