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.

No matter. With such a space craft, Sir Roger can wind up the French war in a week, go on to liberate the Holy Land from the Saracens, and be home in time for the hay harvest. He embarks all of his people, include women, peasants, and livestock, and sets forth. The treacherous alien, though, locks in the automatic pilot to take the ship to the nearest Wesgor occupied planet. The English barbarians, having been lucky so far, will simply have to surrender once the ship lands at the Wesgor star base.

Well, of course, that's not how it happens. With a combination of guile and guts, Sir Roger and his motley crew manage to conquer the galaxy. It's an outrageous scenario that crosses the line of insanity, but Anderson makes it work.

There are a number of subplots. Sir Roger's marriage to a high born lady is troubled and wondering off into the galaxy has not improved things. There's a treacherous, but good looking Welsh nobleman ready to take advantage.

There are jokes aplenty. The negotiations between Sir Roger and a Wesgor Admiral, during which Sir Roger pretends to represent an English Empire consisting of many planets, is a delight of duplicity. The outrageous battle scenes, where English knights defeat futuristic Wesgor soldiers, are exciting and funny at the same time.

The High Crusade ends centuries later when a ship from Earth encounters what the English wrought. They are greeted by soldiers in Lincoln green and armored knights. And the Captain, an Israeli, is able to tell the English descended planetary baron that, yes, the Holy Land has been liberated from the Saracens.

The book was made into a very bad movie directed by Roland Emmerich, who later became more famous for Independence Day and Star Gate. It would very much be a better project in the hands of, say, Terry Gilliam.