Could be, but the problem was they were also highly splintered and (apparently) disorganized. Dozens of tribes with not one liking another, no allegiances apparent at all. Not a hard thing to overcome for a rich nation with a strong tradition of powerful arcane casters and weak infantry.
Not that the Middle East isn't full of compelling examples of apparently fractious tribes doing an awesome job of defying apparently invincible powers, which the Duchy isn't, but DnD and Real Life use completely different scales of time -and that's really the critical element for a battle involving so small a territory. The Wild Elf forest in question isn't all that big, though I can't cite any square X-age off hand, and is bordered by a different openly hostile state and a vast, deadly desert so there's no (again apparent) fall-back position besides a mass grave.