I don't think it's cheating at all. Actually, I think it's pretty cool. Maybe I'm biased because I'm a history buff, but I love reading retellings of things I already know about.
Ex: The show Twin Peaks would reference occult theory a lot, down to namedropping actual people involved. But I definitely didn't feel cheated when I realized it was based on something I already knew about, it was more like I was in on the secret. I actually wrote a whole essay about it for one of my classes.
Anyway, if it's not historical fiction, the outcome isn't going to be one to one with an event. So even if the reader can guess how the battle overall is gonna turn out, they'll probably still be invested in if the individual characters will live.