Is it possible that if France had gone communist, or more likely a hard left side of socialism, it would not have been quit like what we saw in the 1930's USSR, but something much more self-centered? I suspect the French people would think of themselves first, such as workers rights, better working conditions, health, etc... Such me-first movements tend to reduce military budgets while social spending goes up dramatically, for years at least. If we present no threat to others then surely no one will attack us, right?
A French experiment with communism would make Europe less stable as this major power left the world stage to deal with its unhappy citizens. It is hard for me to see the French getting stronger from going communist as any such major change in politics tends to create weakness, even if only temporarily. This weakness for France could possibly be the strengthening of communism in other countries, like Spain, the Balkans, US, and the UK as those movements are motivated by success in France. While these other countries may not fall to communism, it would add to their burdens in countering its increasing influence.
A weakened France, might upend the powers that Europe counted on to contain Germany, or any other nation's, appetite for revanchism. The UK in particular might very well feel threatened from the expanding communist threat and see itself as the last major European power that has not lost its mind. It might start looking inward, too. The British Empire had more than a few movements it was dealing with. Seeing France succumb, could make the UK get very serious about its own issues and feeling very much alone.
What would Germany do then? It might feel much more emboldened to do exactly what it did do up until September, 1939. Hitler's confidence that the old Entente would do little to nothing would only be reinforced. The biggest decision would most likely come between, who do they invade first, Poland or France, because neither could be left standing now. Communism was to great a threat to Germany to let it fester at its borders in the west and Poland was unfortunately in the way as it looked east.
