I am thinking that a French national collapse could in WWI could only be induced by a mobile, blitzing type ground enemy and a sea enemy capable of cutting off men and supplies from the United Kingdom.
The Miracle at the Marne and the French stand at Verdun both demonstrated that not only was French national morale intact, but that even the ubber efficient German army could not force strategic type infantry break throughs.
Battering the French army into national collapse may take some socio political additions:
- Armed, loud and proud anti colonial rebellions sweep the French and UK empires. Morrocans, Algerians, Senegalese, Malagasy and the rest of the French empire are not arriving in mainland France. Likewise, Indians, Nigerians, Ugandans, Kenyans etc will not be feeding into British expeditionary campaigns.
- Britain goes pretty darn isolationist. Only volunteers can serve in rifle Brigades on the western front- and given the death rate at the immediate front and the ability to serve in safer (well usually) expeditionary campaigns, UK volunteers for French trenches start to dry up pretty fast. The end result is much reduced direct UK contribution to France.
- Austria Hungary is stronger and though will not be transitioning to strategic offenses, can hold the line with out continuous German bail outs.
- Verdun, 1916 France stands alone against the uber efficient and reinforced Germans. Things look grim.