alternatehistory.com

https://en.wikipedia.org/wiki/Plan_XVII
The completion of the fortress lines between Belfort and Verdun in the late 1880s and railway building from the interior to the border, gave the French army the means to contemplate a defensive-offensive strategy, in which a German attack would be repulsed and then followed up by a counter-attack. In August 1891, Plan XI was completed, with an option for an offensive as well as a defensive strategy from the start, to exploit the opportunity created by the improvement in relations between the Third Republic and the Russian Empire. The Franco-Russian Alliance (1892–1917) led to Plan XII in February 1892, in which an immediate invasion of Germany was considered possible but from Plan XI to Plan XVI, the main strategy remained defensive-offensive, French attacks being expected after an initial defence against a German invasion.[1]

https://en.wikipedia.org/wiki/Victor-Constant_Michel
Michel correctly predicted the main elements of the German Schlieffen Plan, believing that the major German offensive must come through central Belgium, because of the obstacle of French defences in Lorraine, the terrain in eastern Belgium and German railway building. At a meeting of the Superior War Council on 19 July 1911 he presented these views and proposed a defensive strategy of deploying a million-strong French army on a line Verdun-Namur-Antwerp to counter the anticipated German offensive. In order to field an army of this size French reserve units would need to be integrated with the active army immediately on mobilisation.[1]

Michel's proposal was met with hostility by members of the Government and senior Generals as it ran against the offensive spirit of the Army. The War Minister Adolphe Messimy described it as "comme under insanité" and on 28 July 1911 Messimy replaced Michel with General Joffre and dismissed him from the Supreme War Council. General Michel was then appointed Military governor of Paris.

https://fr.wikipedia.org/wiki/Plan_XVII
Despite the matter of Belgian neutrality, Joffre remained favourable to an offensive, rather than defensive-offensive strategy

https://en.wikipedia.org/wiki/Schlieffen_Plan#Plan_XVII
The instruction of the Commander in Chief was that

Whatever the circumstances, it is the Commander in Chief's intention to advance with all forces united to the attack of the German armies. The action of the French armies will be developed in two main operations: one, on the right in the country between the wooded district of the Vosges and the Moselle below Toul; the other, on the left, north of a line Verdun–Metz. The two operations will be closely connected by forces operating on the Hauts de Meuse and in the Woëvre.
— Joffre[30]

and that to achieve this, the French armies were to concentrate, ready to attack either side of Metz–Thionville or north into Belgium, in the direction of Arlon and Neufchâteau.[31] An alternative concentration area for the Fourth and Fifth armies was specified, in case the Germans advanced through Luxembourg and Belgium but an enveloping attack west of the Meuse was not anticipated and the gap between the Fifth Army and the North Sea was covered by Territorial units and obsolete fortresses.[32]


Let's say general Michel wins the power stuggle, doesn't get sacked and replaced with the idiot Joffre, but instead has his plan for a defensive deployment into Belgium on the Verdun-Namur-Antwerp line become accepted doctrine. WW1 also starts on schedule and Moltke goes with the OTL Schlieffen Plan (because otherwise this is whole thread is pointless).

1. Can the French stop the Germans in Belgium? (My guess is yes)
2. How screwed is Germany if they do? (My guess is a lot)
3. How screwed is France if they don't? (My guess is no worse than OTL, and probably a whole lot less)
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