The collapse of the Fourth Republic proved to be a watershed in Francois Mitterrand's political career. From being intimately connected with the regime, Mitterrand became an isolated outsider, rejected by old and new alike. Far more than any other event, Mitterrand's political career was shaped by his confrontation with de Gaulle in 1958. Opposition to de Gaulle enabled Mitterrand to project a new political identity (as a man of high Republican principle), refine a new political strategy (the union of the Left) and gradually evolve a new personal philosophy (the proclaimed commitment to socialism). His Republican resistance in May-June 1958 set Mitterrand apart from most Fourth Republic politicians, who rallied to de Gaulle, believing erroneously that they could later control the General once the immediate crisis had passed. Mitterrand's stance in opposing de Gaulle demanded undoubted political courage. He was joined by the PCF, by a minority of Socialist deputies, and by a minority of progressive mendesiste Radicals in his opposition to de Gaulle's investiture as premier on 1 June 1958.