In my story a medieval civilization of elves is facing an invasion from a modern day earth power. They have prophetic powers, which have predicted the invasion will happen in ten years, and have that time to prepare.
The earth power is similar to modern day Russia, with a similar airforce and mechanized infantry. The fantasy world power is an empire of elves who are part of a vast empire spread across an area the size of North America.
The elves have technology similar to ours in the 1500s. Their magic grants them food generation power superior to modern day earth, so their population is similar to ours, at around 1 billion. They can also craft wood dwellings that are slightly tougher than native trees quickly, and train pets slightly better than modern day animal trainers can.
From their prophesies, they know that the invasion is coming in ten years, that the invading power wants their vast untapped riches and wealth, isn't open to negotiation and is fine with genocide, and know about nuclear bombs, modern spying technology, poison gas, biological weapons, and the common weapons of doom. Portals of varying sizes will open at random locations across their land and invaders from the foreign land will spew out soon after.
They are fine abandoning their cities if needed, and are politically unified enough to organize a huge amount of manual or violent labour to resist invasion. They have quick communication continent wide by horse back message posts.
Airpower is a major issue. How would they best adapt their civilization to handle such an issue? That is the main question I am curious about. How would a fairly intelligent, coordinated, but vastly technologically inferior species try to mitigate airpower? An obvious basic answer is, attack them at their airbases, but that's obviously quite hard if it's surrounded by machine guns and artillery.
Could they lure airplanes low enough to hit them with siege weapons? Could they damage them with smoke if they burnt a city the invaders were bombing? Could they defeat airbases better somehow? Better answers will explain how a medieval civilization could adapt use preparation time to reduce the power of airpower from a numerically inferior but technologically superior civilization.