The major global event that began with the invasion of Poland in 1939 was World War II. This invasion by Nazi Germany on September 1, 1939, prompted Britain and France to declare war on Germany, marking the official start of a conflict that would last until 1945. The war involved numerous nations and led to significant changes in political, social, and economic structures worldwide, as well as profound human suffering. The consequences of World War II shaped modern international relations and led to the establishment of the United Nations and a new order during the Cold War period.