This cover art was previously used for Novelets of Science Fiction - an anthology edited by Ivan Howard, for both its 1963 and 1967 printings (in paperback.) Attribution confirmed at artist's website: ramaguirecoverart.com

Norman Spinrad's first novel: The Solarians (1966.) 300 years ago the Solarians retreated to the safety of their Fortress as Earth became embroiled in the first of the computer wars with the dread Duglaari Empire. The Solarian's final word to all humanity was a promise to reappear one day and bring it to victory. Suddenly, with Earth on the verge of becoming a helpless victim of the merciless Duglaars, the Solarians made contact with Fleet Commander Jay Palmer. It was an offer of aid. But the Solarian's plan was so fraught with danger that Jay faced the greatest decision of his life - and that of Earth's. Should he accept their strategy as a stroke of genius or reject it as a trick designed to destroy human life forever?