George Brinton McClellan 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.

McClellan organized and led the Union's Peninsula Campaign in southeastern Virginia from March through July 1862. Initially, McClellan was somewhat successful against the equally cautious General Joseph E. Johnston, but the emergence of the aggressive General Robert E. Lee turned the subsequent Seven Days Battles into a humiliating Union defeat.

General McClellan failed to maintain the trust of President Abraham Lincoln. He was insubordinate to commander-in-chief and privately derisive of him. He was removed from command in November after failing to decisively pursue Lee's Army following the tactically inclusive but strategic Union victory at the Battle of Antietam, and never received another field command. 

Most modern authorities have assessed McClellan as a poor battlefield general. After the war, Ulysses S. Grant was asked for his opinion of McClellan as a general. He replied, "McClellan is to me one of the mysteries of the war."

from Wikipedia