Explanation: George McClellan was a Union general during the first half of the US Civil War. While a skilled organizer and well-versed on military theory, he was cautious to the point of absurdity, and let several opportunities to crush the slavers' rebellion slip through his fingers. Most infamously, a couple of Union scouts found and returned the exact written orders of Robert E. Lee, the rebel general, allowing McClellan to launch an attack against Lee with near-total control of the initiative.
... the battle was Antietam wherein the Union suffered extremely heavy losses despite turning the rebels back, and an opportunity to pursue and crush the rebellion was let slip through McClellan's fingers, again, because McClellan believed the rebels secretly had nearly double the troops that they actually did, and would crush him if he pursued.
... they did not.
McClellan was removed from command not long afterwards. The Union would go on to crush the slaver rebellion several years later under General Ulysses S. Grant, who was a much more aggressive leader.