Once there was a world. Bigworld it was called, and aptly so. And in it, life; life lived in it, or rather, on it.
In the beginning of Bigworld there were orcs and elves, who due to their differences in genetics and lifestyle hated each other like the xenophobic prats that they really were, and thus tended to fight a lot. However, orcs and elves tended to live a bit farther from each other, since any orc or elf population in close proximity to each other had a tendency to annihilate one or another, or both. And no matter what they claim as their casus belli, be it demonic possession or vengeance, deep down it was all xenophobia. And why not? Orcs are strong and tough green-skinned brutes who exhibit a strongly r-strategic population growth and a perchance for visceral combat whenever they feel like it, which is very often; elves are graceful and pretty peoples who in their perceived physical perfection meant that they are narcissistic - and quite competitive. Having preconceived notions of the mystical, and abducting lesser races for the required rituals for it, certainly didn't help either.
However, Bigworld is understandably big, and after some time the orcs and elves couldn't fight each other anymore since they don't even make contact in the first place, thus leaving them in seclusion in their own little habitats and ecosystems. This then led to troubles with another growing sentient race, the humans. Humans weren't green enough for the orcs, and weren't perfect enough for the elves. This meant that a large group of orcs would every now and then charge into human villages, smashing apart granaries and homes with sheer upper body strength directed at their stone-and-iron axes, while the elves would abduct a kid, or a family, or an entire town, just to test their particular brand of [often-failing] wood-magicks.
Despite these troubles, over time humanity has expanded and progressed, leaving the orcs and elves to stagnate to a relative lack of civilization but despite this, the two non-human races were still unmatched in battle, with the orcs' natural strength and toughness and the legendary elven grace in handling the bow and arrow being far more effective in warfare than their human counterparts. Indeed, the humans may have survived only because the orcs tended to kill each other if there was no enemy in sight, while the elves had to turn against each other to satiate their interest in magick.
Eventually, the humans have developed firearms, giving them for the first time a chance to reliably defend their cities against raids by elven archers or orcish axemen. The outlying areas however are still unsafe from the marauding orcs and elves because such a feat would require taking the fight to the orcs' and elves' own turf, which was simply impossible. Elves tend to lay and wait in ambush at night at their forest, in one case even sniping an entire human regiment dead in just five hours. On the other hand, orcs tend to live in some pretty inaccessible places; the logistics for lugging that many weapons and ammunition to bring the fight to their communities was too difficult.
Indeed, it was only within the past century or two, with the advent of various technologies such as combustion engines, modern steel industries, and systematic military tactics that the various human nations have become finally confident enough to seriously plan an eradication of these long-time nemeses. The human commonfolk, however, had feared the orcs and elves so deeply down in their psyches that any such proposal would be fiercely resisted by the citizens, nor would any soldiers show up for the expeditions, no. It would take a national tragedy to rivet the people of a country to finally rid themselves of this bane, and face their massive fear of it.