By 1944 the GBL had merged with two other Trotskyist currents to form the Internationalist Communist Party, (PCI), the French section of the Fourth International.
In 1952 he and Pierre Lambert played a key role in the split in the PCI with a minority composed of co-thinkers of Michel Pablo.
The GBL later joined the New Left, within which Bleibtreu created the Revolutionary Socialist Tendency, a founder member of the UGS and the PSU.
He organised in Paris, in 1967, an exhibition named "Art for Peace to Vietnam", a method which he would use again, in 1993, in Athens against the embargo in Iraq.
Bleibtreu supported the development and the coordination of the 'Base Committees' in 1968, endeavouring to reduce the disagreements between the Trotskyist groups, from where his nickname of "Jean XXIII of Trotskyism" comes, following a famous appeal he made in the large lecture theatre of the Sorbonne.