 He then began writing his diary, First Battle, in which he relates his resistance against the Nazis in Chartres, which was later published at the Liberation and prefaced by de Gaulle.Having decided not to collaborate, he left Chartres for Saint-Andiol, Bouches-du-Rhône, to study and join the French Resistance, and he decided to negotiate with Free France.He started to use the name "Joseph Jean Mercier" and went to Marseille, where he met other résistants, including Henri Frenay and Antoine Sachs.