How Do We Explain the Shocking Turn in the Fortunes of War?

In less than six weeks, the french had gone from certain annihilation to capturing belgium, savoy and the constellation of principalities along the rhine. The big overarching explanation is clearly that the allies had underestimated just how resiliant the french would be. A steady stream of aristocrats, high church officials and wealthy families packed up everything they owned and fled east as the french accepted the wide open invitation to move on in.