You can learn more at "Fairy Wars Part 1)
Books often divide fairies into two courts, however, there were often many hundreds, even thousands of fairy courts. In Irish lore, for example, each district had different fairy kings and queens to rule ove them. Thus each land could be said to have it's own fairy guardians who battled the fairies from other lands in order to protect not only themselves, but the people of their lands. According to "Fairy Faith in the Celtic Countries" (Wentz)
The invisible Irish races have always had a very distinct social organization, so distinct in fact that Ireland can be divided according to its fairy kings and fairy queens and their territories even now; and no doubt we see in this how the ancient Irish anthropomorphically projected into an animistic belief their own social conditions and racial characteristics. And this social organization and territorial division ought to be understood before we discuss the social troubles and consequent wars of the Sidhe-folk. For example in Munster Bodb was king and his enchanted palace was called the Síd of the Men of Femen; and we already know about the over-king Dagda and his Boyne palace near Tara. In more modern times, especially in popular fairy-traditions, Eevil or Eevinn (Aoibhill or Aolbhinn) of the Craig Liath or Grey Rock is a queen of the Munster fairies; and Finvara is king of the Connaught fairies. There are also the Irish fairy-queens Cleeona (Cliodhna, or in an earlier form Clidna,and Aine.
As you can see, even the small island kingdom of Ireland had multiple fairy courts, each of which was in near constant competition with the others. Of course each of these kingdoms could be much larger than the human land where they were supposed to be because as I point out in my article on "Elf Kingdoms"
In European lore kingdoms of elves and fairies were often times found in tiny forests. In Shropshire, among other places, for example, any small batch of trees could could contain a fairy kingdom. Most ordinary humans walking through these small batches of trees wouldn't run into the fairies or elves, or even know that there was a kingdom which stretched for miles contained within them. The fairy and elven kingdoms in these woods were in a magical dimension which could only be reached under specific conditions, such as walking counter clockwise around a certain tree or hill.
In Yeat's "Fairy and Folk Tales of the Irish Peasantry" a man named "McAnally tells how once a peasant saw a battle between the green jacket fairies and the red. When the green jackets began to win, so delighted was he to see the green above the red he gave a great shout. In a moment all vanished and he was flung into the ditch."
Human's it seems were very much involved in the politics of the fairy realm, perhaps much like we are involved in rooting for our own local sports teams. There is, however, one difference. When the fairies of one land lost to those of another they would steal the fertility from the other land. Therefor the winners of these wars would have a good harvest, while the losers would have a poor one. We perhaps see remnants of this idea in the rituals of the benandanti of Northern Italy and other Alpine traditions. Here Perchta or some other fairy queen would leave shamans and fairy like beings to do battle with witches who sought to steal the fertility of the land. Such witches were almost always from neighboring villages. It makes sense to presume that such battles might have once been between rival villages for the fertility of the land, to insure that they, not their enemies had a rich harvest.
What we see from this is that humans and fairies were often dependent on each other. This, however, wasn't always the case. In lands as far flung as Ireland and Japan humanity had to battle and defeat the spirits of the earth in order to gain the right to live, hunt, fish, and farm.
In Japan, for example, there were a Kami which appeared as serpents with horns. These kami attacked the people trying to turn a valley into farms. Finally a hero arose who defeated the kami and drove them into the mountain forests. He then made a treaty with them that people would provide them with offerings and prayers in return for the land they'd taken and rich harvests. In Ireland the Druids helped to defeat the Tuatha De Danann, driving them them underground. Among the Komi people of Siberia, the Koreans, and Manchurian peoples, heroes defeated forest and lake spirits in order to force them to allow people to hunt and fish in previously sacred forests.
The idea of humanities war with nature is likely as ancient as humanity.
It makes sense that these wild nature spirits would also become the deities of war. Again Wentz states that;
It is in the form of birds that certain of the Tuatha De Danann appear as war-goddesses and directors of battle, and we learn from one of our witnesses that the 'gentry' or modern Sidhe-folk take sides even now in a great war, like that between Japan and Russia. It is in their relation to the hero Cuchulainn that one can best study the People of the Goddess Dana in their role as controllers of human war. In the greatest of the Irish epics, the Tam Bó Cuailnge, where Cuchulainn is under their influence, these war-goddesses are called Badb (or Bodb) which here seems to be a collective term for Neman, Macha, and Morrigu (or Morrigan) each of whom exercises a particular supernatural power. Neman appears as the confounder of armies, so that friendly bands, bereft of their senses by her, slaughter one another; Macha is a fury that riots and revels among the slain; while Morrigu, the greatest of the three, by her presence infuses superhuman valour into Cuchulainn, nerves him for the cast, and guides the course of his unerring spear. And the Tuatha De Danann in infusing this valour into the great hero show themselves
It shouldn't be surprising that many fertility goddesses or "Earth Mothers" and goddesses in general were more likely to be deities of war than peace. After all the earth is very often associated with the realm of the dead and with human territory. The Swan Maidens of the North were both war goddesses and Grim Reapers, serving either Odin or further east the lord of death Erlik. Among the Kalmyk(the only Buddhist Nation in Europe) the Swan Maiden's closest friend was a tiger in the underworld who sought to devour those who entered her realm.
None of this should be surprising because, just as the Roman's and the Hun's did,, anyone who was more skilled at war would ultimately consume their neighboring cultures. This means that those cultures which believed in war like deities where far more likely to have survived the constant migrations of the past.