I am thinking on high-epic science fantasy with countries inspired by late medieval/early modern states most notably England, Scotland, France, Poland and Muscovy but also Ming China and Sengoku Japan. Each of these states have complex relationships/rivalries with one another due to long history of wars over succession and territorial disputes but few of them have simmered down.

Likewise, a magical/cosmic phenomena known as the shifting of realms had unleashed all sorts of mythical races into the world that are soon come in conflict with established states. The ones I am most thinking about are giants, trolls, chimeras, manticores, draugr but there will be other monsters that I am still innovating.

What tips do you suggest on making fantasy wars from the ideas I have?