I would say that Early's inability to quickly defeat Lew Wallace at Monocracy was it, in 1864. Lew Wallace was the last thing between him and Washington. At this point Washington, in spite of probably being the most heavily fortified place on Earth at the time, had practically no troops garrisoning it. If Early could quickly beat Lew Wallace, he could bag the city pretty easily. However, Lew Wallace (outnumbered almost 3-1) put up a good fight, and in spite of their defeat they delayed Early an entire day. They bought Grant enough time to rush the entire VI Corps to the Capital, staving off Early. If Early had been quicker and taken Washington, it would have been a massive blow for the Union. Would it necessarily have followed that the Rebels would have won the whole thing as a result? Hard to say, but it's the last time I can see a real chance of a truly decisive Rebel victory. At the very least, it would have given the Copperheads a lot more credibility going into the election.