So let's imagine there's the Good Guys. And the Bad Guys. And the war is going on, and then . . . a new horrible menace shows up! And the Good Guys and (previously) Bad Guys join up and learn the value of friendship.
Wait, no they don't. They join up and then the Bad Guys backstab the Good Guys, and the Horrible Menace joins the good guys, and they all completely trounce the Bad Guys.