The famous global conflict that began with the invasion of Poland in 1939 was World War II. Triggered by Adolf Hitler's aggressive expansionist policies and the Nazi-Soviet Pact, Germany invaded Poland on September 1, 1939, utilizing blitzkrieg tactics. This invasion prompted Britain and France to declare war on Germany on September 3, marking the official start of the war. The conflict rapidly escalated, involving multiple nations and leading to significant military operations across Europe, Africa, and Asia. World War II resulted in immense human loss and suffering, reshaped geopolitical boundaries, and led to the establishment of various post-war international institutions aimed at preventing future conflicts.