The elves are a patient people; they know that a war of 50-100 years wouldn’t even span the breadth of elven adulthood, but would be several orcish generations altogether. In that time, orcish leadership would change many times, and the disparate tribes would inevitably revert to their norm, which was chaotic in-fighting that prevented unity, at which point they could be easily thrust back into the traditionally orcish lands.
What the elves could not have anticipated was how the presence of elven breeding slaves would bolster and unify the orcish tribes. Elven women could be bred ceaselessly for decades and become the mothers of a hundred orc warriors, born under no specific tribal banner.
The elven hubris borne of their longevity became their downfall; morale was shattered when elven soldiers saw former comrades and loved ones taken captive years and years prior, now serving happily in their roles as broodmothers and trusted advisors for the orcish war effort. It was a glimpse into their own futures as the momentum of the elf-born orcish army swept deep into the heart of ancestral elven lands…