Imagine if George McClellan hadn't been so conservative, when the Union Army stood outside Richmond. The Rebels had tree trunks disguised as cannons, and McClellan was so conservative that he ignored President Lincoln's order to attack. McClellan had a lot of political backers, and had planned to run for President in the next election. So he felt he could safely ignore the President's instructions.

Of course, he got replaced by U.S. Grant shortly after McClellan's refusal to commence a battle to take the Rebel's Capital City, Richmond, which would have ended the Civil War almost before it was started. So it worked out in the end. But imagine if McClellan had been a competent military leader.