Humans have seen their power ebb and flow over the years. Once nothing, then the mightiest force in the land, now somewhere in between. City-states with different levels of power and influence, some infighting even, different smaller families besides the royal one. Masters of trades thanks to their harbor cities and massive (merchant) fleet. Political wise somewhat isolationistic (Think pre-WW1 USA) at this point because of their many losses.
If there are other elf factions I could see them picked one side or the other or try to stay neutral, but those are smaller details for later.
High and Dark elves would be the two big elven Kingdoms/Empires.
Each of our characters would be a high-ranking member of their respective factions. And both would have a personal hatred for the other, fighting and knowing each other for many, many years before the start of the story. It's personal for them, maybe they don't even care about the war anymore, just about killing besting the other. This drive is also impairing their judgment, they make bad/reckless calls just to get a chance, etc. Over the years in one or both characters, something changed. People died, borders change, culture changes, and so on. But this fight between the two, this 'hate' was their only constant in life, for better or worst. Or how I quoted "But sometimes love feels just like hate".
That's so the basic idea I have at the moment. I probably will come up with a bit more to the other factions over time.