Why did Sartre later try to combine existentialism with Marxism?

After World War II, Jean-Paul Sartre concluded that his earlier existentialist philosophy, which emphasized radical individual freedom, was naive and untenable because it had largely ignored the realities of history and social justice. Reflecting on the war and the German occupation of France, he realized that existentialism needed to account for the concrete social and historical conditions that limit human action.12

In his later work, most notably the Critique of Dialectical Reason (1960), Sartre sought to reconcile his doctrine of individual freedom with a Marxist philosophy of history. While he maintained that Marxism was the only viable philosophy for the modern era, he criticized its "ossified" Soviet form for forcing individuals into predetermined categories rather than adapting to specific, lived circumstances. Through this synthesis, Sartre aimed to create a framework that respected individual freedom while acknowledging that human choices are inevitably shaped by social, psychological, and historical constraints.1234