Which event was a primary trigger for World War II?

The invasion of Poland is recognized as a primary trigger for World War II because it initiated a direct military conflict between Germany and the Allies. On September 1, 1939, Germany invaded Poland using a strategy known as blitzkrieg, characterized by rapid, coordinated attacks combining air and ground forces. This invasion prompted Britain and France to declare war on Germany two days later, marking the official start of World War II.

The importance of this event lies not only in its immediate military consequences but also in the broader context of European tensions following World War I. The failure of earlier diplomatic efforts, such as appeasement towards Germany and the rise of fascism in various countries, set the stage for war, but it was the actual invasion that catalyzed the combat between major powers. Thus, the invasion of Poland is crucial in understanding the timeline and causative factors that led to the outbreak of World War II, making it the most significant trigger among the options listed.