Why Germans would allow French annexing Vallonia? Germans were even in OTL occupying the country during almost whole war.
Most plausible way is divide Belgium during Royal Crisis after WW2.
Different WW1. Perhaps they don't occupy all of the country, the trenches run mostly along the Flandrian-Wallonian border, shifting back and forth a bit.
Here's a quick rundown:
Dutch-Belgian relations become strained in the early 20th century due to a whatever. I would leave this to someone who knows more about the regions politics of the time, I admittedly know very little. The point is, both sides become belligerent, and an incident occurs when a Dutch royal or a politician gets assassinated in Belgium during a visit in the summer of 1914. Crisis erupts, with the British trying to negotiate but to no avail. Eventually the region erupts into civil war, and France supports the Belgian government and moves troops in to restore order. The Netherlands in outraged, saying the French and Belgians are trying to persecute the Flandrian Dutch population. Emboldened by a secret guarantee from the Kaiser, they mobilize to protect the Dutch in Flandria. France and Belgium give an ultimatum for them to demobilize, which they refuse, while sending weapons and volunteers to the Flandrian rebels. Belgium declares war with French support, which prompts a German response, so France declares war on Germany, which leads to Austria-Hungary declaring war on France, which then leads to Russia declaring war on AH, getting a German DOW in return. Serbia follows suit as can be expected. Britain watches in horror as the situation spirals out of control, but since the Belgians started the war, they cannot join, especially since the public opinion is very anti-French, seeing them as warmongers, while the Flandrians as the ones fighting for freedom. Italy stays on the fence.
The French and Belgians start maneuvers by moving into Flanders and Netherlands, then get pushed out by the intervening Germans and the front solidifies as Russia invades East Prussia like in OTL. No British intervention, the Germans crush Russia in the East like IOTL, Austria-Hungary fares better as they only have to defend (no pressure to invade Serbia, can focus on defending Galicia). Russia crumbles by the end of 1916, and Franco-Belgian offensives fail in the trenches of Flanders with terrible casualties. facing riots and outright revolution, Russia sues for peace, as Italy joins the war on the CP side, making a terrible attempt to invade southern France, but gets some success by using torpedo boats against French ships near Nice. Seeing the writing on the wall, France sues for peace as well, while still in a somewhat strong position. Britain arbitrates the peace conference, which is a tough bargain, but ultimately Russia gets a Brest-Litovsk, while Belgium gets divided: Netherlands annexes Flanders, Germany gets some minor border areas and Luxemburg. Germany also gets Belgian Congo and some smaller French colonial territories. France agrees to give up their claims in Alsace-Lorraine, in exchange for annexing Wallonia. While the hawks in Britain are concerned, the liberals argue that German colonial claims have been sated, which is underlined by the Anglo-German Naval Agreement in 1918, greatly reducing tension between the two powers. Austria-Hungary gets its playground in the Balkans, but turns inwards for the next 50 years or so. Germany now has to tend to its new possessions in Africa and has their Mitteleuropa to realise, which will likely become such an industrial juggernaut that Britain might decide to either leave colonial affairs and focus on reorming the Empire/Commonwelath, or try to interfere in a Cold War/Great Game type of skulduggery. Given how the Germans are now so obviously stronger, the French might start a reapproachment in the 1940s, especially if Russia does not get back on its feet or if Britain is reluctant to support them directly (it would be logical for them not to want to interfere in a colonial war directly).
I could go on, but that’s not the aim of the thread
