Final answer:
World War II was triggered by Germany's invasion of Poland in 1939, which led to Britain and France declaring war on Germany.
Step-by-step explanation:
The event that triggered World War II was Germany's invasion of Poland in 1939. This aggressive action was justified by Nazi propaganda claiming that ethnic Germans had been mistreated in Poland. The annexation of Austria (the Anschluss) and the occupation of the Sudetenland were significant preludes to the war, but it was the invasion of Poland that compelled Britain and France to declare war on Germany, marking the beginning of World War II.
The event that triggered World War II was Germany's invasion of Poland. In 1939, Germany launched a military invasion of Poland, which marked the beginning of the war. This invasion was a clear act of aggression by Germany, and it prompted France and Britain to declare war on Germany in defense of Poland.