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The book starts out with human space explorers coming across very tall humans that left earth around a thousand years earlier. Turns out, the tall humans are descended from knights that charged an alien spaceship that landed on earth.

The knights end up conquering the aliens by laying siege to their cities and converting them to Christianity.

marked as duplicate by FuzzyBoots story-identification May 27 '17 at 21:28

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  • How old exactly is the book? Which language did you read it in (and in which country)? Take a look at this page for more guidance on writing a detailed story-ID question. – Gallifreyan May 27 '17 at 18:23
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This is probably The High Crusade by Poul Anderson, originally published in the 1960s, but re-released many times over the years including some versions including the short story "Quest".

It is 1345, and in the English town of Ansby (in northeastern Lincolnshire), Sir Roger, Baron de Tourneville, is recruiting a military force to assist king Edward III in the Hundred Years' War against France. Suddenly, an enormous silver spacecraft lands outside the town. It is a scouting craft for the Wersgorix Empire, a brutal dominion light years from our solar system. The Wersgorix attempt to take over Earth by testing the feasibility of its colonization. However, the aliens, having forgotten hand-to-hand combat since it was made obsolete by their advanced technology, are caught off-guard by the angered Englishmen, who mistake the craft for a French trick. The villagers and soldiers in Ansby storm the craft and kill all but one Wersgor, Branithar.

....

The Crusader arrives at Tharixan in days, and Sir Roger learns of this new world: it is sparsely-populated, with only three fortresses, Ganturath, Stularax, and Darova (the chief base). The humans capture Ganturath but destroy the Crusader in the process. Word spreads of the invaders and a meeting is arranged between Sir Roger and his soldiers and the chief of Tharixan, Huruga.

The humans and Wersgor hold talks that do very little to give either side any advantage, but a truce is agreed to. Sir Roger, in order to intimidate the aliens, makes up tall tales about his estate, "which only took up three planets" and his other accomplishments, including a very successful conquest of Constantinople. Sir Roger demands that the entire Wersgorix state submit to the king of England. During the talks, Baron de Tourneville ignores the truce, and orders the capture of the fortress of Stularax. Unfortunately, the entire base is obliterated by an atomic bomb. In retaliation, Huruga attacks Ganturath again, but loses. He is forced to give up.

....

Sir Roger goes on to topple the Wersgor Empire and build one for himself. He manages with the help of not only the species under the Wersgor, but from members of the Wersgor race who rebelled against their government. The religious figures in the story go on to establish a new branch of the Roman Catholic Church.

In the epilogue (not the beginning):

A millennium after the main events of The High Crusade, the holy galactic empire founded by Sir Roger and his people finally reunites with long lost Earth. A spacecraft from Earth comes across the empire, and is welcomed by the descendants of one of Sir Roger's leading soldiers.

There is, in the epilogue, a reference to events on Earth since 1345. The captain of the Earth ship is described as being a loyal subject of an Israeli empire. It also appears that Huruga wound up as an Archbishop.

The epilogue also establishes that the rest of the book is a translation of a historical text carried by the long-lost Earthlings.

Baen Books has some sample chapters that you can peruse to see if they match up with your memory.

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