World War II began on September 1, 1939, when Germany invaded Poland. This act of aggression prompted Britain and France to declare war on Germany shortly thereafter. The conflict was rooted in unresolved issues from World War I, including territorial disputes and the rise of totalitarian regimes. Over the following years, WWII expanded globally, involving many nations and leading to significant loss of life and destruction. The war would eventually reshape geopolitical alliances and lay the groundwork for the Cold War, ultimately leading to the establishment of institutions aimed at maintaining global peace.