Through the Dark Mirror — Andrew Harvey

After eighty years of total war the remnants of humanity on the Nayarit Line struggle to survive in their sealed domes, surrounded by radioactive wastelands. Now, in the last, desperate years of the war Commissar Traek arrives at Chiqu, a small research facility on the west coast of North America, to investigate a case of possible treason. But the treason is far worse than even he suspected, and as the domes begin to fail and the Empire falls, he finds himself transported from his dying world to a parallel Earth. An Earth that is totally unprepared for the technology of the invading Nayarit. An Earth that will demand from him one final sacrifice.