Hitler threatened the peace of Europe by rearming. In 1934 he wrecked the League of Nations Disarmament Conference. Then he built up his army – at first in secret, then, in 1935, openly holding a huge rearmament rally. Britain even made a naval agreement with Germany, accepting Germany’s right to a navy 35% of the British navy. After 1936, Hitler reintroduced conscription, and poured money into Germany’s armed forces. By 1939, Germany had 95 warships, 8,250 airplanes and an army of nearly 1 million. Germany’s rearmament threatened the peace of Europe because it undermined the treaty of Versailles, and because it forced other countries to rearm and get ready for war. An armed Europe made war more likely. Hitler even war-tested his armed forces in the Spanish Civil War; in 1936 he told his generals to get ready for war in 4 years’ time.
In 1936, Hitler moved his troops into the Rhineland demilitarised zone (although his commanders had orders to retreat if the French army tried to stop them). This threatened the peace, not only because there was a chance that France would fight back, but because it discredited the League of Nations, which did nothing. The remilitarisation of the Rhineland also undermined the peace because it broke the Locarno Pact, a treaty to respect frontiers and not to go to war.
Two more events in November 1936 threatened the peace of Europe. Hitler made the Axis alliance with Italy, and then expanded it with Italy and Japan into the Anti-Comintern Pact – an alliance against communism. Hitler had allies now, and could afford to be more aggressive.
Hitler also endangered the peace of Europe by inciting Nazis in other countries. In 1938 he got Austrian Nazis to demand Anschluss – then he invaded Austria and declared Anschluss (when he had tried in 1934, Mussolini had stopped him by massive the Italian army on the frontier – now Italy was Hitler’s ally). This, too broke the Treaty of Versailles, but it also destroyed the reputation of France and Britain. In 1938, the Austrian Chancellor Schuschnigg had asked Britain and France to help, but they had refused. This (and the west’s ignoring of human rights violations such as Kristallnacht, 1938) endangered the peace of Europe by making Hitler think he could do anything.
In 1938, Hitler got Sudeten Nazis, led by Henlein, to cause trouble, claiming they were being oppressed by the Czechs. Hitler demanded union, and threatened war – and the Czech leader Beneš was prepared to fight. War seemed very close. But then, at Munich, Britain and France gave the Sudetenland to Germany. This was appeasement in action, and it still endangered the peace because it made Hitler think that he could take lebensraum in Poland with impunity.
These actions of Hitler threatened the peace. But it can be argued that it was not just Hitler who undermined the peace of Europe, but also Britain and France, when they allowed – and even helped – him to do what he did.