The armistice with the United States was proclaimed on 8 September 1943. With US and British forces invading from the south and German forces ensconced in the centre and north of the country, a period of intense danger for the civilian population of Rome appeared imminent, and Gorresio was one of many who went into a form of hiding. In his case, this meant quitting his editorship at Il Popolo and moving in with a distant cousin, the historian , whose public profile was less exalted than his own. He tried to earn some money by undertaking academic research: for instance, he edited the so-called "Opuscoli politici" ("literally, "political leaflets"") of "M. Taparelli d'Azeglio" (1943). That assignment led him to become a regular visitor to the Rome central library, which was more discretely located at that time than it is today, and which had become the semi-clandestine meeting point for other intellectuals doing their best to stay out of sight. Here he was introduced by the partisan-journalist Felice Chilanti into a group of writers who called themselves "Armata garibaldina" ("loosely, the "patriotic army""). Within the group Gorresio took for himself the informal pseudonym "fantomatico". Using it, he edited and distributed a little clandestine news-sheet called "Azione".