After the Battle of Antietam, McClellan failed to capitalize on Lee's retreat.

Despite having achieved a strategic victory, McClellan hesitated to pursue the Confederate Army as it withdrew across the Potomac River. This inaction allowed Lee to regroup and escape without further engagement, missing a crucial opportunity to potentially deal a more decisive blow to the Confederacy. McClellan’s cautious approach and failure to exploit Lee's weakened position drew criticism, as it ultimately prolonged the war and allowed the Confederates to maintain their fighting capabilities.