Between his first philosophical works and his last, Jean-Paul Sartre radically changed his philosophical outlook. The reasons for this change can be found in European history and Sartre's detailed study of twentieth-century protest movements. Between the end of the Second World War and the 1960s, French intellectuals began an intensive period of introspection, examining the complex relationship between History and social justice. Sartre and the group of intellectuals associated with him combined to fight against Stalinism while searching for a new theory of political action. This thesis discusses the abrupt termination of the ethical project that Sartre proposed to base on his original phenomenological examinations, and discusses his and Simone de Beauvoir's first attempts to construct an Existentialist ethic. Sartre changed from being an Existentialist to a Marxist to finally, late in life, abandoning Marxism in favour of a never well-defined philosophy. But in the Critique of Dialectical Reason, the last of his serious philosophical works, he responded to his ex-friends's critiques in the light of his study of Eastern European history, particularly, the Revolution in Hungary. (Abstract shortened by UMI.)