Triplanetary by E. E. Smith

One of the newest and fleetest of the patrol vessels of the Triplanetary League, the heavy cruiser Chicago of the North American Division of the Tellurian Contingent, plunged stolidly through interplanetary vacuum. For five long weeks she had patrolled her allotted volume of space. In another week she would report back to the city whose name she bore, where her space-weary crew, worn by their long "tour" in the awesomely oppressive depths of the limitless void, would enjoy to the full their fortnight of refreshing planetary leave.

She was performing certain routine tasks -- charting meteorites, watching for derelicts and other obstructions to navigation, checking in constantly with all scheduled spaceships in case of need, and so on -- but primarily she was a warship. She was a mighty engine of destruction, hunting for the unauthorized vessels of whatever power or planet it was that had not only defied the Triplanetary League, but was evidently attempting to overthrow it; attempting to plunge the Three Planets back into the ghastly sink of bloodshed and destruction from which they had so recently emerged. Every spaceship within range of her powerful detectors was represented by two brilliant, slowly-moving points of light; one upon a greater micrometer screen, the other in the "tank," the immense, three-dimensional, minutely cubed model of the entire Solar System.