World War II began in Europe on September 1, 1939, when Germany invaded Poland, prompting Britain and France to declare war on Germany shortly thereafter. This invasion marked the start of widespread conflict across Europe and soon expanded into a global war involving numerous countries. Under Adolf Hitler's leadership, Nazi Germany sought to establish dominance, leading to a series of aggressive military campaigns and the eventual involvement of major powers like the United States and the Soviet Union. The war would reshape global politics, economies, and societies for decades to come.