1936 with the militarisation of the Rhineland. The Wehrmacht literally had orders to turn around and run back to Germany if they were opposed.
The question then becomes whether the French can a) have the will to follow up with an invasion of Germany and b) convince their population and the international community to go along with it. The answer is probably sadly no to both.
However, Hitler may well fall as a result of the Rhineland humiliation. He might not, but he might.