This is from an exchange I had with David R on the RPG Site. He mentioned that his IHW: Napoleonic Naval game was historical except for gender roles - i.e. they had an openly female captain. He added that he supposed that this made his campaign not historical. My reply was that all historical games become alt-historical once the PCs set foot in them, though some are more historical than others. This is a very important point, and one which could go a long way towards easing the apprehension most gamers feel towards running historical games.
It's like the man who asked a woman if she'd sleep with him for a million dollars. She thought a bit, and said she would. He then asked if she's sleep with him for twenty dollars. She got very angry and asked if he took her for some kind of whore. His reply was that they had already established that, and were now dickering over the price. By allowing PCs into an historical setting, you have established that it is non-historical, and now you are just dickering over how non-historical it's going to be.
GMs in particular fear running historically based games because they don't want to mess with history. They prefer something which is alt-historical from the get-go, because then they can let the players have their head. This can lead to what I call the "Inglourious Basterds" school of historical gaming, where they go completely over the top, because as long as they are being non-historical, anything goes. BTW, I'm not knocking the movie, as I loved it - it was my favorite this year, and the best thing Tarantino ever did. :D
Other GMs, if they do allow their players into an otherwise historical setting, try to keep them to the cracks and crevices of history, places and times where not a lot is known, and nothing important is happening. Thi "PCs as Historical Roaches" approach has never really appealed to me, and it seldom appeals to players. They prefer to be where things are happening, important things, where they can take a hand in forming what comes next.
That's a big point in my experience! Players love making history. My approach is to keep it as historical as possible, except for the PCs' actions, and the reactions to those actions. They were not there historically, but they are there in the game. It's their presence that makes this alt-history, and they can change it however they wish, so long as they make their rolls, and work with the materials that are there. If they want to kill Hitler, then they are welcome to try. It ain't going to be easy - many, many people tried to assassinate Hitler, and they all failed - but it will be possible.
So the question is not "Is it historical?" The question is "How historical is it?"