In 1926 he became secretary of the Confédération Générale du Travail (CGT: General Confederation of Labour) union of postal workers in the Lyon region.

[1] During the Munich Crisis in September 1938 Belin endorsed Édouard Daladier's policy of appeasement and revision of the Treaty of Versailles.

At the November 1938 national meeting of the CGT in Nantes Belin obtained the support of one third of the attendees for a pacifist and violently anti-communist resolution.

[1] Georges Bonnet, together with his allies in the "peace lobby" both within and without the government such as Anatole de Monzie, Jean Mistler, Marcel Déat, Paul Faure, Paul Baudouin, Pierre Laval, René Belin, Adrien Marquet, and Gaston Bergery, all spent 1–3 September 1939 lobbying the Daladier government, the Senate and the Chamber against going to war with Germany.

[2][3] During World War II (1939–1945), after the defeat of France in June 1940 Belin said it was necessary to completely rethink unionism within the national framework.

He developed the Charte du travail (Charter of Labour), a compromise between retaining some autonomy for unions and complete "corporatization" of the economy in the Fascist model.

The movement was joined by former members of the Confédération des syndicats professionnels français, a union created by François de La Rocque in 1936.