Which event is considered the starting point of World War II for the UK?

The invasion of Poland in 1939 is recognized as the starting point of World War II for the UK because it was this event that prompted Britain to officially declare war on Germany. On September 1, 1939, Germany invaded Poland, which violated the Anglo-Polish military alliance. This act of aggression established a clear threat to European stability and security, leading Britain to take decisive action. The declaration of war occurred just two days later, on September 3, 1939, illustrating how pivotal the invasion of Poland was in triggering the conflict that would engulf much of the world.

Other options include events that occurred later in the war or had different implications. For instance, the invasion of France in 1940 was a significant military campaign but happened after the war had already commenced. The attack on Pearl Harbor occurred in December 1941, which was the catalyst for the United States' entry into the war rather than the UK’s initial involvement. The signing of the Pact of Steel in May 1939 represented an alliance between Germany and Italy but did not directly lead to the start of hostilities.