When two fairy clans went to war with each other the one with humans on their side would win. This is because it was believed that humans had many natural magical powers, powers which allowed us to over come the fairies, even if we aren't aware of these.
These natural powers which all humans have to some extent or another, fall into four categories.
1-The ability to use magical objects, herbs, symbols, or words that fairies can't.
So while the magical beings of many lands are driven away by herbs such as garlic, elder wood, soy beans, or objects such as iron, humans can utilize all of these objects freely. In fact The power to work, manipulate, and use iron was unique to humans across Eurasia. In many cultures from Europe to Japan Iron could not only hurt fairies, kami, and many other magical beings, it could strip them of their power. Even a tiny child could stop a powerful spirit by throwing an iron knife at it, an act which would break its power.
Further humans could draw symbols or say words that would break a spirits power. Early on, in parts of Europe, pentagrams were believed to keep spirits away, and while the symbol later changed to the cross, the general idea was the same. That humans had power over symbols which could drive away spirits.
Indeed humans would draw these crosses on tree stumps in Germany to protect the wood wives inside the tree from The Wild Huntsmen. The woodwives, couldn't draw these crosses on trees, they needed humans to do it in order to stay safe.
2-The ability to utilize the evil eye.
Humans were believed to have a natural evil eye. The human gaze could drain a fairy of their powers, such that as long as a human was looking at a leprechaun, for example, it couldn't use it's magic to vanish, change shape, or do anything else. This is how humans were able to capture fairies.
Some people had such powerful evil eyes that they could kill with a look.
This is likely the reason that fairies remained out of human sight as much as possible, as it was painful to have some humans look at them. Or for humans to see them under certain circumstances.
3-The ability to withstand polluting influences
Urine, saliva, blood, etc. Are all polluting influences which would drain spirits of their magic. This was especially true in Japan where kami possessed rocks, trees, animals, etc. In order to avoid encountering these polluting influences. Further , humans saliva was poison to the monsters which would hunt mountain kami, so only a human could defeat them, by liking their weapon.
It was also true in Europe and native Alaska, among other places, to a lesser extent.
4-The natural ability to resist the fairies magic.
Fairies often had trouble touching or hurting humans except during specific times or circumstances. Thus as long as humans avoided certain taboos the fairies would often have difficulty affecting them.
So even if the fairies wanted to kidnap someone they often couldn't unless that person walked around a sacred tree three times, or did something else that would give the fairies power over them.