No sudden, shocking invasion, nor great decisive battle that won a great empires. Not glorious war with renown to be gained. No, the slow grim march of decades of lost territories. Border skirmishes. Siege. Fallen forts. And always the growing cold.
For the invaders have time. They can afford to wait.
Elves are the shadow in the north. They first began taking our lands in my grandfather's time, bringing war with their servants and allies, the wolves and the frost giants.
Orcs are their soldiers. Using foul magics, they created orcs by combining goblins and men. Orcs are unnatural things, not of this world. They appear as men during the dark of night, walking among us with their wicked knives. The sun reveals them for what they truly are.
Goblins hate them as abominations and hate the elves for making them. Cruel, wanton, and often unpredictable, goblins are still man's greatest ally in the fight against the elves.
Halflings love comfort. They are collaborators, gladly trading with the elves and living in their newly conquered lands as good citizens. Not volunteers, mustering in the armies -- just quiet, simple folk. Good citizens who don't want trouble and are willing to look the other way when life gets uncomfortable for those who choose not to play along.
Dwarves wander through any land they must in pursuit of their obsessions -- gold and dragons. Dwarves emulate the dragons, worshipping them as gods and gathering hoards for their kingdoms. A great enough hoard will bring a dragon. It is said that in the greatest of dwarf halls, dragon kings sprawl on great glittering hoards, ruling as god kings under the mountain. Dwarf heroes who find dragons risk death stealing into their hoards to draw them to their own kingdoms.
Dragons are forces of nature -- powerful, ancient, and cunning. They do not speak. They do not think as men do. They are fiercely greedy.
This is a start. The idea for this campaign is not to stray too far from the classic D&D building blocks. Elves are still elves, dwarves are still dwarves, halflings are still hobbits. The blocks are just arranged in a slightly different way. These descriptions above are very brief and suggest monolithic groups, but in fact I'll be adding a lot of factions within each group as I develop the setting. Some lands conquered by the elves have adopted their ways. Human rulers and warlords may be in open war against the elves, may be acting as stewards of elf controlled lands, may be quietly plotting to retake their territories, and so on. Others have learned from the elves and are taking new lands of their own, without the elves' blessings, while some human warlords are simply taking advantage of the disorder to carve out territories of their own. Some have learned from the elves only to turn against them, fighting them back and taking back land once ruled by the elves themselves. Similarly, some elves work to oppose the conquest. Even some halflings have chosen to fight back.
There will be room for PCs to align themselves as they see fit.