Complex sentence: The French, all but masters of Spain in June, now stood with their backs to the Pyrenees, clutching at Navarre and Catalonia, not knowing whether these two footholds could be maintained in the face of a Spanish attack.
Split into simpler sentences: The French, all but masters of Spain in June, stood with their backs to the Pyrenees, clutching at Navarre and Catalonia. It was not known if even these two footholds could be maintained in the face of a Spanish attack.