A number of reasons. First, by this time French military policy had essentially given up on the notion of defending the Rhineland. Instead, French military planners had thrown everything into the construction of the Maginot Line, which they imagined would be the basis of a defensive war against German aggression. For that reason too, French military forces lacked the capacity for a speedy mobile response to the German reoccupation. The French also greatly exaggerated the strength of the German military, further incentivizing their defensive posture behind the line.

At the same time, the British were even more reluctant to punish the German reoccupation. According to the treaty that now governed Rhineland demilitarization--the Locarno Pact--German hadn't done anything "flagrant" to violate its terms, since it did not seem to be a precursor to an attack on France itself. In reality, the British government had no stomach for a war, and British rearmament (only recently begun) focused on defending the British Isles rather than projecting British military assets onto the continent.

​