The Germans were stopped from Paris in WWI. Then they were pushed back into a band of land 50-75 miles wide and kept there in a stalemate for years until their defeat.
The only reason they were stopped was because they exposed their flank to Paris. The Germans were then in an unattainable position.
How did Churchill put it? I cannot remember exactly, but when the allies moved forward they “seridipudiously probed their way into the German liver”.
Read the histories of those there. The French commander (Joseph Jaques?) could hardly believe his luck when the Germans turned. Heck Jaques had already issued his stand and die command by this point.
This is also when the Jaques begged French (the English commander, I know confusing) that they had to strike now. French did not want to as his force was tiny, but he understood the opportunity at hand. Jaques then replied he was waiting to see if England still had any honor.
While we never know what might have been, the fact of the matter was, had the flank opportunity not presented itself, the French were in a very bad position. It is also important to note that each side was at the very end of endurance. When Jaques ordered the attack his subordinates were appalled. They had men sleeping on their feet they were so tired.
But they rallied and the rest is history.