It would seem to me, that your main problem would come from Germany not succeeding in hurting France sufficiently in the opening stages of the war.
Here are some thoughts on that campaign from my experience so far.
As Frank states taking the 3 resource hexes near Verdun (actually 4 resource hexes) helps CP a lot, while hurting Entente production.
If Germany chooses a France first it is imperative to hurt French manpower from the beginning, and where possible to destroy some of the 18 at start infantry corps, which constitutes 18 out of a total of 20 category A corps for France. France does mobilize 10 corps on turn two, but still does not get the necessary rail movement to move them into position.
Germany can concentrate approximately 900 strength points against 400 for France. Force the French to maneuver and attack them in the open with your superior forces.
With the addition of AH artillery set up in the alps at mobilization and using strategic movement to get to the front in the first impulse, CP will have an astonishing 9 artillery pieces to bleed France with.
Furthermore, go for poison gas to help blasting the lines. Remember to keep 1 research point on turn 1, because you won't get further research points until turn 3.
Note that France only get 36 arms on each of the first two turns and starts with 80 manpower, receiving 40 manpower a turn. So they are vulnerable to attrition.
Hurt French manpower from the beginning and even when switching to defensive keep hurting French manpower.
In my experience, France will have passed exhaustion level 1 in the summer of 15, and what survives of category A infantry corps until then will now be category B corps and the remainder category C corps.
A final note for consideration. It is not necessarily in CP's interest to attack France through Belgium. Not only do you have to fight the 3 Belgian corps but also instantly the 3 British corps. Some artillery must also be allocated, together with ample reserves, and you may not even take Liege on the first impulse.
BEF can enter France on turn two whether Belgium is neutral or not, but France still lacks the necessary rail movement, so they will have to walk to the front
Now, i don't suggest to never attack through Belgium just to be flexible and unpredictable
One last thing though, sooner or later Belgium will join the Entente.