World War II in Europe officially began in 1939 when Germany invaded Poland on September 1st. This aggressive action prompted Britain and France to declare war on Germany on September 3rd. The invasion of Poland marked a significant escalation in hostilities that had been brewing in Europe since the rise of totalitarian regimes and unresolved issues from World War I. As a pivotal event, the invasion led to a series of military campaigns and the eventual involvement of numerous countries in a global conflict that would last until 1945.