Der große sieben fünf

Though WWII actually started in Asia, when Japan invaded Manchuria in 1931, we'd be remiss in not noting the 75th anniversary of Germany's invasion of Poland on September 1, 1939.  Which would not have happened if Neville Chamberlain and Edouard Daladier had stayed home in September 1938, rather than fly to Berchtesgaden and ceded part of Czechoslovakia to Hitler.  As Deutsche Welle notes;

What is tragic about the events around this time in history was that, as of September 1938, Hitler was very alone with his plans for war. His generals wanted to avoid a war at any cost. Chief of the German General Staff Franz Halder, who was a top commander in and around Berlin, along with Berlin's chief of police had already formed a new government with civil service workers critical of the NS and former Social Democrat politicians. A secret brigade of assault troops was prepared to overrun the Reich Chancellery as soon as Hitler declared war.

That anti-Nazi coup was to be triggered by Hitler's order to invade 'the Sudetenland' (largely populated by ethnic Germans), which the Generals in the German army knew they were being trained to do. With the appeasement of Hitler though, that order didn't have to be given. Further, that England and France were exposed as feckless changed a lot of German officers' minds, and a great deal of German opposition to Nazism disintegrated.

Instead, today we celebrate(?) the fruits of that decision by Chamberlain and Daladier.