World War II began on September 1, 1939, when Germany invaded Poland. This military aggression prompted the United Kingdom and France to declare war on Germany, marking the onset of a widespread conflict that would last until 1945. The war involved multiple nations across Europe, Asia, and beyond, driven by various political, economic, and social factors, including the rise of fascism and militarism. World War II resulted in significant loss of life and unprecedented destruction, reshaping international relations and leading to the establishment of the United Nations in an effort to prevent future global conflicts.