That is one of the main issues, and one Kaisereich never really answered to my satisfaction.
Nonetheless, a defeated Britain at the end 1918 can't really do much to France, they are exhausted and have a great many colonial commitments plus Ireland. OTL they were badly over-stretched by the early 1920s, and I'm not certain a defeated Britain would be much better able to keep its men in uniform, particularly when you also have hostile German colonies to defend against etc.
All around, the RN can blockade France I'm sure, but I'm not sure the UK is in a position to do too much on land.
Germany has major problems too, given the war exhaustion, the Russian Civil War, garrisoning the colonies, settling the Middle East. If the French Revolution really kicks off after Germany has overextended herself in Russia I'm not sure they can find too many men to secure control over France. Italy also strikes me as likely to collapse, though I've always envisioned an Italian Social Republic as more likely to emerge from that havoc I would expect there would be communes and anarchists aplenty there as well before it settled down, so Germany could get busy there too.
I'm not saying that a Communist France is all that likely, I don't know enough about French internal politics in this period to know what would happen (I would be interested in recommendations for good English books on the subject however) but I do think the international situation could potentially allow for all manner of French regimes to emerge, depending of course upon what was actually possible given domestic conditions in France.