Times Without Number — John Brunner

If the past is tampered with, the present might be totally transformed. So the whole fabric of reality depends on the watchful efforts of the Society of Time. Don Miguel Navarro is a junior officer in this force dedicated to defending the Spanish Empire and the mother church from the results of meddling in history by time-travellers.

But he begins to wonder just how dedicated the Society really is when he has to deal with a case of corruption involving fellow officers . . .

After he has to rescue the entire court from death at the hands of Amazon warriors brought through time, his greatest trial becomes unavoidable. Facing a threat to the most vulnerable event in his world's history, can the young Don prevent catastrophe?

Or will the glorious triumph of the Spanish Armada never have occurred?

(First published 1969)