Answer:
General McClellan’s most grievous error was hugely overestimating Confederate numbers. This delusion dominated his military character. In August 1861, taking command of the Army of the Potomac, he began entirely on his own to over-count the enemy’s forces. Later he was abetted by Allan Pinkerton, his inept intelligence chief, but even Pinkerton could not keep pace with McClellan’s imagination. On the eve of Antietam, McClellan would tell Washington he faced a gigantic Rebel army “amounting to not less than 120,000 men,” outnumbering his own army “by at least twenty-five per cent.”
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