"If He had executed judgment upon them, but not upon their gods, it would have sufficed."
(The Passover Haggadah)
In our own world, as might very well be in the different fantasy worlds, religion can be one of the primary reasons to go to war. In a fantasy campaign world, the classic chain of events starts with a god (or demigod, or lesser god, or deity, or immortal - feel free to change the terminology to fit your favorite campaign world) deciding, for his own reasons, to declare a war on another god. He commands his followers to beat down their plowshares to swords and embark on a holy war against the followers of the other god. The outcome of the war in the mortal world often determines the result of the celestial war too. The winner gains glory and the loser loses a large portion of his might and influence, and sometimes even his life.
Another element of classical fantasy is that a god derives his might from his followers and their devotion, which could create an interesting magical circle in such a situation. Many of the losing god's followers leave him since he has proven himself weak by being defeated by another god. And on the other hand, having the number of his followers diminished only serves to furthermore weaken him.
What always bothered me with this notion is the idea that only one cosmology governs the world. The gods of the campaign setting belong only to a single pantheon, who's makeup and power balance are well known (or at least, well known to all those who take active interest in it - clerics, priests, scholars and other celestial beings). However, in our own world, that was only rarely the situation. More often than not, wars were fought between different people who held completely different beliefs. The Celts and the Romans clashed in dozens of battles when the Romans occupied Gaul, and alongside these battles, a culture war raged across Gaul. All of this took place without the Celts recognizing the Roman gods or the Romans recognizing the Celtic gods.
How can such ideas be incorporated into our campaigns? How can be broaden our campaign from a family feud inside a given pantheon to a full scale culture war?
First of all, we must ask ourselves how we perceive the gods and deities of our campaign setting. Are they true and unique Gods, the personification of their powers and their source, or are they "merely" omnipotent and immortal cosmic beings?
If Mars (the Roman God of War) is the personification of War itself, obviously Teutates (The Celtic God of War) cannot really be a God of War. Assuming this is how are campaign looks, there are a few interesting possibilities regarding Teutates' existence.
He might not be a god at all, just a superstition. In this case, he will just wither and disappear into oblivion if defeated, but what would happen if his followers actually won? Would Mars disappear and a new God of War be born out of the void?
Alternatively, Teutates could be an actual deity, albeit less powerful than Mars, the real God of War. In this case, Mars would probably come down with his terrible wrath upon this imposter if he wins, but if Teutates wins he might take Mars' place - not only as a title, but also as cosmic force - he would become the living personification of the concept of war, which might prove to be a bigger bite than he can swallow.
A third option is that Mars and Teutates are in fact the same being, only given different names and appearances by different people believing in him. In such a case, it is highly unlikely that the two sects of followers would go to war (since they both serve the same god anyway, he would gain nothing from them fighting each other). In the unlikely case such a fight actually takes place, its outcome may serve to modify the characteristics, appearance, etc. of this god.
A more interesting point of view might be if the gods are not personifications of cosmic powers like described in the previous paragraph, but simply very powerful and ancient beings. In such a case, any one of the optional results I described in that paragraph may occur, but another interesting result is also possible - merging of common elements.
In the real world, even the most overwhelming military victory didn't always (or more accurately - almost never did) result in a victory in the cultural war taking place in the conquered territory (for instance - Judaism survived to this very day, despite Israel being conquered by Babylonian, Greeks and Romans). Alongside closing local temples and legalizing the practice of local religion, one of the most common ways to eliminate the local religion is the merging of elements.
Basically, the idea is to take the points of similarity between the losing and the winning religion and to emphasize them at the expense of the points of difference. This way, the losing religion (or at least extensive parts of it) are merged into the winning religion and the losing religion stops existing as an independent religion. The Romans were excellent mergers (consider the fact that almost every Greek god has a parallel Roman god, but the Christians weren't below it either (it isn't a coincidence that The Devil in Christian myth is described as having goat horns, exactly like Ollathir, the Celtic all-father). Even Judaism, the first and original monotheistic religion the tale of creation is a clear reference to Enuma Elish, the Mesopotamian creation tale.
In a campaign setting where the Gods are really influenced from the beliefs of their flock there are several interesting situations to consider.
Ollathir's surprise when he discovered he is changing to the personification of all that is wrong and evil may be great, but not half as much as Mars' and Teutates' who would find themselves being merged to one being, each one of them previously the master of his realm having to share not only his power with the other, but his very essence.
One last suggestion: Read the four issue mini-series Marvel vs. DC/DC vs. Marvel from 1996 and the Amalgam line from 1997. Although they deal with superheroes and not gods they accurately depict a war of pantheons and element merging.
This article first appeared in issue #5 of The Orc.