Which military action by Germany marked the beginning of World War II in Europe?

The invasion of Poland is recognized as the pivotal event that marked the beginning of World War II in Europe. On September 1, 1939, Germany launched a surprise attack on Poland utilizing a new military tactic known as Blitzkrieg, or "lightning war," which emphasized rapid and coordinated assaults using infantry, tanks, and air support. This invasion prompted Britain and France to declare war on Germany just days later, on September 3, 1939, as they had pledged to protect Polish sovereignty. The swift conquest of Poland demonstrated Germany's military capabilities and set the stage for further aggression in Europe, leading to the expansion of the conflict into a larger world war. This event is thus seen as the critical starting point, distinguishing it from subsequent military actions that occurred later in the war.