What event marked the beginning of World War II?

The invasion of Poland by Germany on September 1, 1939, is recognized as the event that marked the beginning of World War II. This action triggered Britain and France to declare war on Germany, as they had pledged to support Poland in the event of an invasion. The aggressive expansionist policies of Nazi Germany under Adolf Hitler were embodied in this invasion, which demonstrated a blatant disregard for international treaties and norms established after World War I.

The other options do not represent the onset of World War II. The attack on Pearl Harbor, which occurred on December 7, 1941, was a significant event that brought the United States into the war, but it was not the beginning. The signing of the Treaty of Versailles in 1919 aimed to establish peace after World War I and set many of the stage conditions for World War II, but it did not initiate the conflict. The fall of Berlin occurred in 1945 and marked the end of the war in Europe, not its commencement. Hence, the invasion of Poland is the pivotal event that set off the wider conflict of World War II.