I'd have gone with an alien force kidnaps a small flotilla from the midst of the second world war but it consists of both allies and enemies who are initially intent on killing each other until their kidnapper announces that they have been selected to fight against another fleet this one from an alien race with the homeworld of their opponent up for grabs!
The allies would still fight each other but an alliance between the two enemies leave them no choice but to face off against a technologically advanced foe with whatever means they have and for this I see the japanese finally use their kamikaze attacks to weaken the enemy who have no idea such a tactic exists and a mixed force forcibly boards the enemy battleship and succeeds in winning the battle but when the surviving allied ships return the entire situation is covered up so that the Japanese never realise how they helped save the world and the "allies" begin work on reverse engineering the tech retrieved from their surviving ships...however as a closing sequence we find the heroes who forcibly boarded the alien battleship have survived but were left stranded as they remain on the alien world where we learn the reason the aliens were actually fighting was to flee their homeworld thats being threatened by an nasty alien invader and the heroes find themselves having to protect themselves and their former adversaries against this new foe...
Good enough for a sequel wouldn't you say?
