Listening to various podcast, notable The Napoleon Quarterly I am quite confused how Valmy is presented by historian like Blaufarb, he claims that the Prussians had no line of communication at that Dumouriez inserted himself just on those lines and cut the Prussians off.
I see it quite the other way round, Braunschweig tried to cut off and out manoeuvre the French Army - which he did. Only by the unhappy interference of the Prussian King, orders a march to the right, a rechtsabmarsch, there the enemy supposedly was positioned in the west. When the Prussians realized their error, the French patriots are in the East, at the left flank of the Prussians it is decided to advance up to the road of Chalons sur Marne to cut off the French from their line of retreat.
So not the French are cutting of the Prussians, but the other way round.
Goethe in typical fashion was acting as post mortem know it all - when he wrote in 1822 that here and then he witnessed a decisive point in history.
I agree with Charles Esdaile that in case the Prussians would have pressed home their attack they had a very good change to inflict a defeat to the patriots, who showed on several occasions prior to Valmy being prone to panic.
So I cannot see that Dumouriez outflanked the Prussians and cut them off, what he succeeded in, was to concentrate his forces, but in the end he had to fight a battle against the Prussians interposed with his army and Paris.