Galactic Patrol by E.E. Smith

Helmuth sat at his desk, thinking -- thinking with all the coldly analytical precision of which he was capable.

This Lensman was, in truth, a foeman worthy of his steel. The cosmic-energy drive, developed by the science of a world which the patrol did not know existed, was Boskone's one great item of superiority. If the patrol could be kept in ignorance of that drive the struggle would be over in a year; the culture of the iron hand would be unchallenged throughout the galaxy. If, however, the patrol did manage to learn the secret of power, to all intents and purposes unlimited, the war between the two cultures might well be prolonged indefinitely. This Lensman knew that secret and was still at large, of that he was all too certain. Therefore, the Lensman must be destroyed. And that brought up the Lens.