Gettysburg

Gettysburg was a really big deal, because it pretty much decided the outcome of the Civil War. It mattered a lot since it was the Unions first major victory, it was able to get their spirits back up. This gave them hope, so they were more enthusiastic in fighting now. 

If the South HAD won, the Union would have probably given up. Even if they wanted this with all their life, it wasn't going anywhere up to then, so you kinda get tired of trying knowing its not really going anywhere. And it would've been ok to stop their and give up, because there is that point where you've just had enough. Up until Gettysburg their fighting had been pointless, just a waste of time and effort. But Gettysburg was able to stop them from quiting, and they came back stronger.