Why did George B. McClellan lose Lincoln's confidence as a commander?

President Abraham Lincoln lost confidence in George B. McClellan primarily due to the general's persistent reluctance to engage the enemy and his failure to decisively defeat Confederate forces. While McClellan was a gifted organizer who successfully built the morale and efficiency of the Army of the Potomac, he was frequently paralyzed by what Lincoln called "the slows".12

McClellan consistently overestimated the size of opposing Confederate armies, leading him to avoid offensive maneuvers and remain inactive for long periods. During the Peninsular Campaign, his extreme caution prevented him from capturing Richmond despite being in a strong position to do so. Even after successfully stopping Robert E. Lee at the Battle of Antietam, McClellan failed to pursue and destroy the retreating Confederate army. This repeated inability to press his advantages and his refusal to take the offensive ultimately exhausted Lincoln’s patience, resulting in his permanent removal from command in November 1862.12