In 1965 the French left-wing parties joined forces and lent support to their common presidential candidate François Mitterrand. However several left-wing candidates ran for President in the following election in 1969. What were the reasons for suspending the unification process? Was it the occupation of the Czechoslovakia or student riots in 1968? Could that be ideological divergences or personal conflicts between French left-wing forces? The article is based on analysis of press releases, political science research made in the 1960s and observations of the Czechoslovak embassy in Paris.