Triplanetary

The Arisians, benevolent humanoids who have declared themselves Guardians of Civilization, war with the Eddoreans, shapeless, malevolent beings, hungry for power at any price. They fight on both physical and mental levels, wielding weaponry of inconceivable destructiveness.

And their battleground is a tiny planet in a remote galaxy: Earth. The swamping of Atlantis, the fall of Rome, the wars that wrack the world and blaze through space - all may seem historical accidents to the men involved, but each in reality is a move in a save universe-wide power struggle . . .