(we are definetely in a "what-if", not in the debate about the primauty of spanish theater)

I am sure that I read in Oman a letter of Wellington, then on the border of France in front of a beaten, understrength french army (before Soult could give it some flesh), who wrote to an english official that he would not enter France before the events in Germany became more clear.

Strategically, it makes sense : before wandering in South Western France, Wellington could not anticipate the attitude of the french population. To enter deeply in the country, with possible thick fog of war and patriotic guerillas, to become the target of a triumphant Armée d'Allemagne, was certainly not a choice.

So, imagine his decision if Napoleon had twice more strength on the german front...

All the more, France even in 18e century is asymetrical in roads, population and industry : under a line from Rouen to Marseilles, there is not many strategic targets. And Auvergne mounts are a shield for the Loire, Seine and Rhône valleys. So an invasion from Navarre would take much time to be decisive, and would have long, vulnerable flanks. The kind of opportunities that the Duke did not offer to his adversaries.

Best regards,

Jean