George Brinton McClellan (American National Biography)

As commander of the Army of the Potomac, he was perhaps the only man in the country who could have won the war in an afternoon, but he was also one of the few who could have lost it in that span. The accomplishment of denying so formidable an opponent as Lee his goal of crushing the Army of the Potomac in the Seven Days' battles was not without merit, but its credit to McClellan was interred long before the general's bones were laid to rest. … Although on at least two occasions he had the power to reach forth his hand and end the war during the summer of 1862, timidity palsied that hand, and the killing went on for another two and a half years. Ironically, his failure of nerve ensured the adoption and full implementation of the hard war policies he deprecated, including emancipation. His squeamishness for the lives of his soldiers doomed them to added years of combat…Gifted with a brilliant intellect, McClellan also had personal charm and a remarkable ability to win the affection not only of his soldiers but of nearly everyone who came into contact with him…His ability as an organizer and motivator of troops was equally impressive.

He seemed unable or unwilling to apply his remarkable intelligence in new ways or in the face of unforeseen circumstances. His plans often had much merit to them, but he lacked the tough-mindedness to see them through to victory.