The allies initially occupied the Rhineland under the Treaty of Versailles but Britain evacuated troops in 1926, followed by France in 1930. In March 1936 Hitler moved German troops into the Rhineland, breaching the Treaty of Locarno. Neither France nor Britain was in a position to prevent remilitarisation.
Germany was found to be in breach of the Locarno Treaty by the League of Nations, but no action was taken. Belgium reacted by claiming neutrality and withdrawing from obligations. This materially weakened the Locarno guarantees and presented problems for France, because the fortifications of the Maginot Line had not been extended into Belgium. Terms of the Locarno Treaty were renegotiated at a five-power conference.
In April 1936 talks between Britain and France failed despite the build up of military capacity, and they proceeded separately. Due to economic difficulties in 1937 Britain abandoned plans for a continental expeditionary force. The failure of Britain and France to act encouraged Hitler's aggressive policy and was a key event leading up to the Second World War.