So I have generic fantasy Human Empire, relatively Good folks, but mostly Orderly. Racially mostly homogeneous, some elves and dwarves are present, but 99% humans. And there is the Monster Empire, which is ruled by dragons, but has all kinds of naturally Evil monsters (and by Evil I mean DnD like alignments, so tangible Evil), from goblinoids to ogres and whatnot. Pre-Threat the two sides were in equilibrium: the monsters were never united enough to pose a significant threat to the otherwise also quite powerful Human Empire (lots of magic users and a strong army + various paramilitaristic organizations), and the Monster Empire is located in a hard to pass terrain, so a formal army's movements are extremely hindered. The monsters range from threat to only unskilled or old/wounded/ill/children to the mentioned dragons, which pose a significant threat to whole groups, though not unbeatable by any means. Border skirmishes were regular before the alliance.
They both got undeniable proof that there is some Generic Threat that threatens the very existence of the world, and the lead dragon bands together with the Human King to stand togetherish against the threat.
The world is a band of land from west to east, bound by mountains and waters. Monster Empire is to the west, Human Empire is in the middle, and the Threat comes from the east.
The thing is, the threat is yet to materialize. What are some key points I would have to keep in mind when thinking about this? I would imagine the dragon would keep a sizable force in Human Empire, because of logistics, if the threat arrives (and they have no clear idea of how long it will take from the first actual sign to the invasion), they have to be there. So right now, there is a whole family of dragons (Evil, macho, aggressive), a good number of giant sized creatures (ogres, trolls, actual giants) and a sizable array of smaller goblionoids, orcs, kobolds and other cannon fodder.
Is there any kind of historical reference for this? That might be a good basis for it, sans the Evil monsters part.