plotting strategies

So, it's been a while since I came on this platform. And honestly, I don't do this enough to have the confidence, nor do I know if this is the right place to put this, but it's a pain not have anyone to talk about my story with, so here goes. So, my fantasy novel (which has heavy Lovecraft...

I have a war that's referenced fairly often in the fantasy series that I am currently writing, as it's the main influence of events that happen in the series, and I need some help with fleshing it out in greater detail for my personal reference. I have no idea how to plan a war or write one...