Turning enemy agents into double agents for the British

The Nazis and their Spanish allies initially seemed to have had the advantage in the secret war for Gibraltar. However, the British were able to use the "Double Cross" technique - that of identifying enemy agents and "turning" them into double agents for the British authorities - just as effectively in Gibraltar as they did in Britain. 

DSO staff identified many hostile agents and either arrested them or turned them into double cross agents. No successful act of land-based sabotage occurred after July 1943, once the "screen" erected by the DSO's double agent network was in place. A total of 43 sabotage attacks on the naval base were forestalled through the use of double cross agents. Sea-borne attacks remained a problem; a number, including several carried out by Italian frogmen, were successful.