The Freedom Army — E.C. Tubb

To escape war and suffering, humanity had allowed itself to take a drug that killed all emotion. But a small number of insurgents - including the returning astronauts from a mission to Mars - had refused to take the drug, and waged an armed rebellion. The last survivors of the Freedom Army were holed up in a besieged bunker when one of their number, the physicist Burges, constructed a gateway to another dimension-an alternate existence. Ex space-pilot Lanson leads the remnants of his Freedom Army in escaping through the gateway-only to find themselves in an alternate world were humanity has been enslaved for the last 30 years by a toad-like alien race, the Zytlen!