What is an impossible memory? The Algerian war has been described as an impossible memory - meaning?

The statement concerning the Algerian war by Robert Gildea that you cited is not a factual statement, it is an opinion, a viewpoint. Possibly he is making that statement as a reaction to someone he talked to who was involved in the fighting of that war, or a civilian victim of the war. Ask yourself what it means when someone describes an event that you have felt in your heart as "an impossible memory." Write down what comes to mind.