In Scenarios, someone found an exploit or hack to betray their team

Not sure how, but I was in a WWII game and China, which is supposed to be allies, attacked me when I was USA.

Well if the cities were empty he can just take them so that might be what happened. If you want to prevent things like that just make sure to leave 1 unit in the city... Or did it actually show up in the battle phases and he really attacked you?

Maybe axis won in China and you never knew?