The French Communist Party (Parti Communiste Français- PCF) was banned in 1939 at the outbreak of WWII when it opposed the war.
Since the USSR and Nazi Germany had a neutrality pact, the PCF may have actually sabotaged arms production. Threatened with execution, the PCF leadership fled abroad.
When Germany invaded Russia in 1940 the PCF formed the National Front within the broader French resistance movement and organized direct action and political assassinations through the armed Francs-Tireurs et Partisans.
By the end of German occupation in 1944, the party, now powerful in many parts of France, entered into the governing Tripartite alliance with the French Section of the Workers’ International and the Christian Mouvement Républicain Populaire.
PCF strength peaked in France at the end of WWII. Gaining parliamentary representation in successive elections, the PCF promoted strikes and opposed colonialism.
In 1947, midst rising concern regarding Communist influence, the PCF was excluded from the French government.
Under pressure from Moscow, the PCF withdrew from other parties and focussed on agitation within its trade union base.
Source: Britannica.com