It’s like this, see. You’re a member of an advanced race, and the
Prime Directive won’t let you use advanced weaponry on the primitive
inhabitants of the backward planets you want to exploit for raw
materials. No, you have to bargain with the primitive little creatures.
And then one of your competitors has a bright idea. They kidnap an army
from a primitive planet–the creatures call themselves “Romans”–and they
make these “Romans” fight their battles for them, using the primitive
weapons they know best. It’s been a couple of centuries, and your
competitor is now top dog. What do you do?

If you’re like the alien in this book you go back to the same planet, and
see if you can find some Romans of your own. What you end up with is an
army of English longbowmen on the way to Agincourt, and everything seems
to go amazingly well–for awhile. But Englishmen have minds of their
own, don’t you know.

This is a fun book. It isn’t Poul Anderson’s
delightful The High Crusade, but it’s fun.