The High Crusade by Poul Anderson

The High Crusade, published over forty years ago and rarely out of print since, is science fiction grand master Poul Anderson’s story about how some aliens try to invade the Earth—and then kind of wish they hadn’t. The aliens land in Merry Olde England in the year 1345, in the barony of Sir Roger de Tourneyville, a some what down on his heels, but starwart nobleman who is in the process of gathering an army to help King Edward III conquer the French.

The aliens, serving the Wesgorix Empire, are well skilled at conquering primitive planets. They land at some inhabited place, overawe the natives with their superior weapons and technology, and take over. Unfortunately for them, they have never invaded the English before. The aliens, in the midst of overawing the primitives, get perforated by a volley of cloth yard shafts courtesy of the local longbow men, led by a merry giant right out of Robin Hood named Red John Hamewood. Sir Roger, seeing that these demon-like alien can actually be killed, draws sword, cried, “Saint George for England!”, and leads his knights and men-at-arms to take the alien space craft.

That is only the beginning. What follows is a kind of 'Connecticut Yankee in King Arthur's Court' in reverse. The saintly Brother Parvus—another character right out of Robin Hood—teachers a surviving alien Latin. The alien has some strange ideas about cosmology. Sir Roger can accept the idea that the Earth is a sphere that revolves around the sun as the Greeks had similar notions. But the stars being other suns? Obviously the alien is having them on. Almost certainly he comes from a land from beyond Cathay.