World War II officially began on **September 1, 1939**, when Germany invaded Poland. This invasion prompted Britain and France to declare war on Germany two days later, marking the start of a massive global conflict. The invasion demonstrated Germany's military aggression and its expansionist ambitions under Adolf Hitler. This event set the stage for widespread warfare in Europe and the eventual involvement of numerous nations worldwide. The consequences of this conflict were profound, leading to significant geopolitical changes and the establishment of international organizations aimed at preventing future wars.