George Brinton McClellan (December 3, 1826 – October 29, 1885) was a major general for the Union during the American Civil War and the Democratic presidential nominee in 1864, who later served as Governor of New Jersey. He organized the famous Army of the Potomac and served briefly (November 1861 to March 1862) as the general-in-chief of the Union Army. Early in the war, McClellan played an important role in raising a well-trained and organized army for the Union. 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.