One of the cooler ideas in Spellforce is that Light and Dark races will fight each other even if they're in the same army. In other words, let's say you're leading a group of human soldiers to assault an orc camp. Figuring that it might work to hit your enemy from both sides, you bring in a group of dark elves round the other side of the camp. As both groups approach the orcs, the battle begins with each of your forces attacking the camps from different directions. But once your human and dark elven soldiers spot each other, the dark elves will leave off attacking the orcs and start attacking your human soldiers instead.