Rhineland
In October, 1933, Adolf Hitler withdrew Germany from the League of Nations and claimed he had done so because of the failure to reach agreement about disarmament. Hitler argued that under the terms of the Treaty of Versailles Germany was militarily weak. He said that Germany had been willing to keep to this state of affairs if other countries disarmed. As this had not happened, Germany now had to take measures to protect herself.
In the months that followed, Hitler trebled the size of the German Army and completely ignored the restrictions on weapons that had been imposed by the Treaty of Versailles. By 1935, when it was clear that no action was going to be taken against Germany for breaking the terms of the treaty, Hitler felt strong enough to introduce military conscription.
Adolf Hitler knew that both France and Britain were militarily stronger than Germany. However, he became convinced that they were unwilling to go to war. He therefore decided to break another aspect of the Treaty of Versailles by sending German troops into the Rhineland.
The German generals were very much against the plan, claiming that the French Army would win a victory in the military conflict that was bound to follow this action. Hitler ignored their advice and on 1st March, 1936, three German battalions marched into the Rhineland.
The French government was horrified to find German troops on their border but were unwilling to take action without the support of the British. The British government argued against going to war over the issue and justified its position by claiming that "Germany was only marching into its own back yard."
Primary Sources
(1) Adolf Hitler met Kurt von Schuschnigg, the Austrian Federal Chancellor on 12th February, 1938. Schuschnigg later recalled what Hitler said to him at the meeting about marching into the Rhineland in March, 1936.)
Don't believe that anyone in the world will hinder me in my decisions! Italy? I am quite clear with Mussolini; with Italy I am on the closest possible terms. England? England will not lift a finger for Austria. And France? Well, two years ago when we marched into the Rhineland with a handful of battalions - at that moment I risked a great deal. If France had marched then, we should have been forced to withdraw. But for France it is now too late!
(2) In his memoirs Lord Halifax attempted to justify his appeasement policy that culminated in the signing of the Munich Agreement in September, 1938.
The criticism excited by Munich never caused me the least surprise. I should very possibly indeed have been among the critics myself, if I had not happened to be in a position of responsibility. But there were two or three considerations to which those same critics ought to have regard. One was that in criticizing the settlement of Munich, they were criticizing the wrong thing and the the wrong date. They ought to have criticized the failure of successive Governments, and of all parties, to foresee the necessity of rearming in the light of what was going on in Germany; and the right date on which criticism ought to have fastened was 1936, which had seen the German reoccupation of the Rhineland in defiance of treaty provisions.
I have little doubt that if we had then told Hitler bluntly to go back, his power for future and larger mischief would have been broken. But, leaving entirely aside the French, there was no section of British public opinion that would not have been directly opposed to such action in 1936. To go to war with Germany for walking into their own backyard, which was how the British people saw it, at a time moreover when you were actually discussing with them the dates and conditions of their right to resume occupation, was not the sort of thing people could understand. So that moment which, I would guess, offered the last effective chance of securing peace without war, went by.