Worse still, because fairies were fertility spirits which gave life to the land their wars and deaths would reshape the very land and throw the country off balance. The potato famine in Ireland was that killed millions of people was said to be caused by a fairy war which disrupted nature and the fertility of the land. In this case people could see the fairies flying over the land going to war with each other. Another fairy war left the world awash with so much blood that the moss where the battle took place turned red.
So what do fairies fight wars over? The same things humans fought wars over. Land, food, honor, a desire for power, to kidnap women or men or because their women were kidnapped. Fairies also fight wars to protect and help humanity. "The War of the Trees" was a liminal war between fairy beings in the other world (including Arthur) which was meant to obtain a golden hind and dog for humanity. What purpose these served humans isn't clear given that the poem is a fragment meant to remind people of something they already knew when it was written. Still, what's key is that the fairies lead an army of trees into the underworld in order to win treasure for humans.
One interesting fact about fairy wars for fantasy writers is that fairies often needed humans to help them with their battles. In Japan, for example, Mountain Kami would often ask humans for help with their battles because humans had the ability to defeat certain things they couldn't touch (kami are made weak by the presence of unclean things such as blood, urine, centipedes, etc).
In Wales the fairy lord Arawn asked King Pwyle to help him with his war against Hargan. In this story Pwyle and Arawn switch places for a year, each pretending to be the other. After ruling the fairy realm for a year "the time for the battle in single combat between Powell and Hargan had fully come. The two warriors met in the middle of a river ford, and backed their horses for a charge. Then they rushed furiously at the other. Powell's spear struck Hargan so hard, that he was knocked out of the saddle and hurled, the length of a lance, over and beyond the crupper, or tail strap of his horse. He fell mortally wounded upon the ground."
This story isn't too surprising considering that it was believed that "when the fairy tribes under the various kings and queens have a battle, one side manages to have a living man among them, and he by knocking the fairies about turns the battle in case the side he is on is losing."
Joseph Jacobs has a fairy tale about a man named Paddy O'Kelly who finds himself in fairy land.He was ultimately led by this lesser fairy court to the high fairy court of King Finvara and Queen Nuala here he was greeted warmly with Finvara who tells him "We are going to play a hurling match to-night against the fairy host of Munster, and unless we beat them our fame is gone for ever. The match is to be fought out on Moytura, under Slieve Belgadaun." The story goes on to point out that; "it is necessary for the fairy host to have two live men beside them when they are fighting or at a hurling match, and that was the reason that little Donal took Paddy O'Kelly with him. There was a man they called the "Yellow Stongirya" with the fairy host of Munster, from Ennis, in the County Clare.
They were hurling away, and the pipers playing until Paddy O'Kelly saw the host of Munster getting the strong hand, and he began helping the fairy host of Connacht.
The Stongirya came up and he made at Paddy O'Kelly, but Paddy turned him head over heels. From hurling the two hosts began at fighting, but it was not long until the host of Connacht beat the other host.
Then the host of Munster made flying beetles of themselves, and they began eating every green thing that they came up to. They were destroying the country before them until they came as far as Cong. Then there rose up thousands of doves out of the hole, and they swallowed down the beetles.
That hole has no other name until this day but Pull-na-gullam, the dove's hole.
When the fairy host of Connacht won their battle, they came back to Cnoc Matha joyous enough, and the king Finvara gave Paddy O'Kelly a purse of gold, and the little piper brought him home, and put him into bed beside his wife, and left him sleeping there.
Because of the strong fairy emotions what begins as a sporting event turns into a serious battle in which hundreds if not thousands of fairies die.
The fact that fairies need humans in order to be victorious in battle leads us to one of the most important battles in fairy history, that in which the Sons of Mil (humans) defeated the Tuatha De Danann and drove them underground. Once underground the fairies began to exercise some control over humanity by becoming the gods of fertility and the harvest. They also continue to occasionally wage war with humans. Typically such wars are fought when the humans take some piece of important land from them or the fairies kidnap the wrong woman and so anger a man who has the resources to wage war on them. When going to war with the fairies there are three important points to remember;
1-Human druids, holy men, and wizards are frequently more powerful than the fairies. Indeed it was through the power of the druids that the fairies were defeated in the first place.
2-Iron hurts fairies and can break their magic.
3-Fairies can't repair damage done to their hills and castles if salt is put on these. In this way particularly farmers have been able to successfully wage war on the fairies in return for their wives.
More often fairies attempt to alter the outcome of human wars, choosing the side they want to win and aiding them in victory. Thus treaties with fairies were extremely important if one wished to survive wars with other humans. In this way the fairies were able to rebuild their power base from behind the scenes.
There is a tradition among the Glamorgan peasantry of a fairy battle fought on the mountain between Merthyr and Aberdare, in which the pigmy combatants were on horseback. There appeared to be two armies, one of which was mounted on milk-white steeds, and the other on horses of jet-black. They rode at each other with the utmost fury, and their swords could be seen flashing in the air like so many penknife blades. The army on the white horses won the day, and drove the black-mounted force from the field. The whole scene then disappeared in a light mist.
There is another interesting point about the fairy armies which is that over time they began to include the ranks of human ancestral spirits. It seems that when humans die their souls went into the fairy hills and became, in essence, fairies. They were then under the command of one fairy king or another. This likely means two things; firstly the fairies have grown in strength since their initial defeat. However, at the same time this increased strength comes from the spirits of humans who care about their decedents and hate the kingdoms they were enemies with in life. This in turn likely explains part of the reason why fairies become so involved in local politics.
As always I'd like to leave fantasy writers with a couple of writing prompts;
1-The fairies kidnap a farmers wife, children, or cattle as is common. This time, however, the farmer decides he isn't going to take it and so he declares war on a small hill of fairies leading to a kin based feud similar to the one between the Hatfields and McCoys
2-Fairies from a neighboring distract attempt to steal the fertility of the land in order to make their fields more fertile. The local fairies declare war in order to defend the human farms and seek human help.
3-Tell a story of two fairy clans which declare war on each other as a point of honor which has nothing to do with their desire to preserve a forest. Bonus points if you can make this interesting without involving humans.