Vast, churning alternate-world/alien-invasion saga. In 1942, as WW II engulfs the Earth, down from space come the reptilian-alien ``Race,'' whose fleetlord, Atvar, has orders to conquer the planet and add it to the Empire. The Race, known to the humans as ``Lizards,'' are an old species, with evenly developed but not particularly advanced technology; their hereditary Emperors have ruled for thousands of generations. Expecting an easy victory over sword-wielding primitives, the Lizards are appalled at how rapidly human technology has advanced. Though their tactics are inflexible, and they learn slowly, the Lizards have nuclear weapons and are prepared to use then (on Berlin, on Washington). Turtledove (A Different Flesh, Agent of Byzantium, etc.) takes a global approach, mingling real and fictional characters, developing a dozen or more occasionally connecting plotlines. A sampling: Major Heinrich Jaeger, sent to invade Russia, instead turns his panzers against the new invaders from space; pilot Ken Embrey of RAF Bomber Command contends with Lizard jets and guided missiles; in the Warsaw ghetto, Moishe Russie first welcomes the Lizards as deliverers, only to discover their real intentions; Red Air Force pilot Ludmila Gorbunova flies across half of Europe, with foreign minister Molotov as her passenger; physicist Jens Larssen struggles to unlock the secrets of nuclear fission in the University of Chicago labs; aboard an orbiting spaceship, Chinese peasant woman Liu Han finds herself the subject of strange sexual experiments. Intriguing and panoramic but circuitous and uncompelling, and lacking even a token ending: will best suit alien-invasion/WW II buffs happy to settle in for a meandering journey of unspecified duration.