Children of the Lens by E.E. Smith

The Arisians and the Children of the Lens had known that Eddore must be attacked as soon as possible after the fall of Ploor. They were fairly certain that the interspatial use of planets as projectiles was new; but they were completely certain that the Eddorians would be able to deduce in a short time the principles and the concepts, the fundamental equations, and the essential operators involved in the process. They would find nth space or one like it in one day; certainly not more than two. Their slaves would duplicate the weapon in approximately three weeks. Shortly thereafter both Ultra Prime and Prime Base, both Klovia and Tellus, would be blown out of the ether. So would Arisia -- perhaps Arisia would go first. The Eddorians would probably not be able to aim such planets as accurately as the Arisians had, but they would keep on trying and they would learn fast.

This weapon was the sheer ultimate in destructiveness. No defense against it was possible. There was no theory which applied to it or which could be stretched to cover it. Even the Arisian Masters of Mathematics had not as yet been able to invent symbologies and techniques to handle the quantities and magnitudes involved when those interloping masses of foreign matter struck normal space.