Japan cannot win, but there is damage and holding the Americans objectives that give in essence a Japanese victory. Really the Axis cannot win in a Global Axis & Allies game but the scenario must warrant situations where they can achieve some sort of victory. I have played Okinawa, both sides, and it was fun whether I played the Japanese or Americans. As the japanese, I made the AMerican pay for every inch of Island they advanced on.
maybe selling the game on the premise of playing 2 games (one as each side) then comparing relative losses for 'who won' would be an interesting way to do it. if its set up where the axis CAN'T win then its essentially, who can damage the allies the most. There are a lot of video games like this, left4dead2 comes to mind first for me, you play both sides, with whom ever did best as survivors winning the round.