This would be a giant clusterfuck. A TL to really fuck up Europe prior to WW1.
Lets just say that a few radical nationalists (maybe Schlageter) kill some French soldiers and bureaucrats in a guerillaesque fashion.
The French army, like most armies at the time, would have been completely overwhelmed. This would have likely caused heavy handed actions hurting and killing civilians. IOTL 137 civilians died, lets just say ITTL this number is higher. 700-800 Germans are summarily executed only stiffening the resistance.
Reichswehr and paramiltitary formations start operating and the death toll on both sides rises. Sure, the Germans don't have the munition supplies for a serious campaign but they have the necessary reserves and manpower to turn this whole affair into something equivalent to a war.
Neither Great Britain nor the USA would help the French, especially not if the civilian casualties are steadily rising.
What are France and Belgium supposed to do? Send their armies deeper and deeper into a hostile Germany?
Hold on... wasn't the german army pretty much stripped of heavy weapons and aircraft by this point? If so, what would these rioters have? Rifles, some MGs and mortars, mayeb some leftover light cannons?