Following the German-Italian-Bulgarian occupation of Greece during World War II, Grivas founded and led the Organisation "X", a minor far-right guerrilla organisation made up of officers of the Greek Army whose influence was limited to certain neighbourhoods of Athens. It played a minor role in the Greek Resistance to the Axis occupation of Greece. Its activities included spying for the Allied powers and launching minor attacks and sabotage operations against the occupiers. During the events of December 1944, members of Organisation X, using weapons recovered from the retreating Germans, fought at the Theseon alongside British and Greek monarchist forces to take control of Athens from EAM/ELAS fighters.