Fifty days to doom

The galaxy is at war between the oxygen-breathing Terran Federation of worlds and the Ginzoes, chlorine-breathing aliens. An Earth ship is captured in battle by the Ginzoes, and its crew learns that the aliens have a newly developed catalyst. This can liberate chlorine from the sea - which, for the Ginzoes, would convert Terran worlds into suitable environments. The catalyst will be used unless the Terran Federation declares peace within fifty days. Fifty days to save Mankind ... or fifty days to doom!