Hitler's attack on Poland can be called as the trigger to World War 2.
Step-by-step explanation:
Hitler was appeased by United Kingdom and France in 1938 by the Munich Agreement which gave Sudetenland to Nazi Germany, and in return Nazi Germany promised not to make any more territoral claims. But in March 1939, German forces occupied the rest of Czechoslovakia. After that event Britain and France gauranteed Poland to go for war for its protection. Hitler began invasion of Poland on Sep 1, 1939. In response, Britain and France declared war on Germany on Sep 3, 1939 and the rest is history.