François Mitterrand built up a resistance network, composed mainly of former POWs. The POWs National Rally (Rassemblement national des prisonniers de guerre [fr], RNPG) was affiliated with General Henri Giraud, a former POW who had escaped from a German prison and made his way across Germany back to the Allied forces. In 1943 Giraud was contesting with de Gaulle for the leadership of the French Resistance.
From the beginning of 1943, François Mitterrand had contacts with a powerful resistance group called the Organisation de résistance de l'armée (ORA), organised by former French military personnel. From this time on, François Mitterrand could act as a member of the ORA, moreover he set up his own RNPG network with Pinot in February and he obtained funding for his own network. In March, François Mitterrand met Henri Frenay, who encouraged the resistance in France to support François Mitterrand over Michel Cailliau. 28 May 1943, when François Mitterrand met with Gaullist Philippe Dechartre [fr], is generally taken as the date François Mitterrand split with Vichy. According to Dechartre, the meeting on 28 May 1943 was set up because "there were three movements [of Résistance:] […] the Gaullist, the communist, and one from support centers […] hence I was assigned the mission to prepare what would be called afterwards the merger [of the three movements]."
During 1943, the RNPG gradually changed from providing false papers to information-gathering for France libre. Pierre de Bénouville said, "François Mitterrand created a true spy network in the POW camps which gave us information, often decisive, about what was going on behind the German borders." On 10 July François Mitterrand and Piatzook (a militant communist) interrupted a public meeting in the Salle Wagram in Paris. The meeting was about allowing French POWs to go home if they were replaced by young French men forced to go and work in Germany (in French this was called "la relève"). When André Masson began to talk about "la trahison des gaullistes" (the Gaullist treason), François Mitterrand stood up in the audience and shouted him down, saying Masson had no right to talk on behalf of POWs and calling la relève a "con" (i.e., something stupid). Mitterrand avoided arrest as Piatzook covered his escape.
In November 1943 the Sicherheitsdienst (SD) raided a flat in Vichy, where they hoped to arrest François Morland, a member of the resistance. "Morland" was François Mitterrand's cover name. He also used Purgon, Monnier, Laroche, Captain François, Arnaud et Albre as cover names. The man they arrested was Pol Pilven, a member of the resistance who was to survive the war in a concentration camp. François Mitterrand was in Paris at the time.
Warned by his friends, François Mitterrand escaped to London aboard a Lysander plane on 15 November 1943 (piloted by then-Squadron Leader Lewis Hodges). He promoted his movement to the British and American Authorities, but he was sent to Algiers, where he met de Gaulle, by then the uncontested leader of the Free French. The two men clashed, de Gaulle refused to jeopardize the Resistance by including a movement that gathered information from POWs. Later Mitterrand refused to merge his group with other POW movements if de Gaulle's nephew Cailliau was to be the leader. Under the influence of Henri Frenay, de Gaulle finally agreed to merge his nephew's network and the RNPG with Mitterrand in charge. Thus the RNPG was listed in the French Force organization from spring 1944.
François Mitterrand returned to France by boat via England. In Paris, the three Resistance groups made up of POWs (Communists, Gaullists, RNPG) finally merged as the POWs and Deportees National Movement (Mouvement national des prisonniers de guerre et déportés [fr], MNPGD) and Mitterrand took the lead. In his memoirs, he says that he had started this organisation while he was still officially working for the Vichy Regime. From 27 November 1943 Mitterrand worked for the Bureau central de renseignements et d'action. In December 1943 François Mitterrand ordered the execution of Henri Marlin (who was about to order attacks on the "Maquis") by Jacques Paris and Jean Munier, who later hid out with François Mitterrand's father.