Belgium after World War II was split over the Royal Question, and this took on aspects of right vs left and Flemings vs Walloons. There was a serious chance of Wallonia seceding. Anyway, if you got past the title, I'm hoping you already know this.
My question (which I ask here because r/althistory is frequented by too many fools) is what would have happened to the Belgian Congo had this happened. This is not intended as a counterfactual, I'm just curious what the exact procedure would have been. Would it have remained a colony of rump Belgium? Could Wallonia legally raise any sort of claim? Did France have a right of preemption over the area? What about Rwanda and Burundi?
Frankly I don't think Wallonia would have stand a chance seceding. While Flanders was pretty much in every district in favor of the return of Leopold this was not the case for Wallonia. The industrial parts were heavily against but the more rural parts (Ardennes, Luxembourg, parts of Namur) were for and Brussels with Brabant was pretty much 50/50.
As to the colonies. There's no way the world powers would have let Wallonia keep them (and what would they do with it: Wallonia is landlocked and of the three harbors that had good connections to the industry cities two are in Flanders and the third one was Rotterdam which they could only reach through Flanders.)
Rwanda/Burundi/Congo would have probably been given to the U.N. (Ruanda-Urundi was a UN Trust Territory already anyway) who would have given mandates to France and the UK