He was assigned to the military office of the secretary of state for aviation in Vichy but contemporaneously was a member of the Ronsard-Troine resistance network.
He actively participated in negotiations during the American landing and went to Morocco and Dakar to rally French forces to the Allies' effort.
In October 1954, in the Revue Militaire d’Information, he wrote of the “ideological role” of the army, its mission of “moral recovery” and guardianship of values.
The aim of the plot, after an uprising in Algeria and France - in which Chassin would seize the Saint-Étienne arms factory and take control of Lyon - was a Catholic, corporatist right-wing state inspired by Portugal's existing Estado Novo.
[12][13][10] He ran in the legislative elections of November 1958 in the 2nd constituency of Gironde against future prime minister Jacques Chaban-Delmas but was well beaten.
[16] In January 1960, having tried to reach Algeria during the right-wing insurrection known as La semaine des barricades, he was arrested by the police but released shortly after.
In 1957, his dismissive opinions on the perceived threat to the West after the success of the Soviet Union's Sputnik satellite were widely reported.
[20] The following year, he provided the preface to Aimé Michel's work A propos des soucoupes volantes - about flying saucers - in which he wrote: "We can therefore affirm that there really are, in the sky around us, mysterious objects" adding that their identification as distinct from enemy missiles was essential given the US reaction to Sputnik.
[21][2] Chassin married Marcelle Momard and had three children, Max, Pierre - also an extreme-right-wing political activist who became an elected official of the Front National[22] - and Claude.