Odds of Getting In
If you work for an exhibit or attraction directly, it'll probably be a little easier to break into this career. They may even teach you some of what you need to know for the gig. After all, you may have a degree in American history, but are you going to know what's on every headstone at Arlington National Cemetery? Of course not—those specifics come once you're learning on the job.
If you want to work for a tour company, you're most likely going to need to know your stuff at least fairly well when you apply. The boss will care about his or her company's reputation, so they'll only want to hire people who know what they're talking about—and can communicate it in an interesting and totally not condescending way.