The Defiant Agents

Humans being what they are will always find an excuse (usually rather stupid) to war with each other. Such is the case for this plane of existence that the story is based on. Alien spacecraft have been discovered by western and Russian forces. The aliens who built and operated them have mysteriously vanished, but left large storehouses behind on various planets. To give their explorers a better chance of survival, both sides reach into their past for the most hardy warrior types, Apaches for the West, Mongol Tartars for the Russians. Not content with volunteers, both sides use mind control devices on their own people to ensure they act as desired - ah. but what happens when those devices don't work? A great tale by the Dame of science fiction/fantasy, Andre Norton.