World War II started in 1939, officially beginning on September 1 when Germany invaded Poland. This aggressive move prompted Britain and France to declare war on Germany on September 3, marking the beginning of a widespread conflict that would last until 1945. The war involved many nations across the globe and was characterized by significant battles, political shifts, and extensive military strategies. It resulted in profound changes in global politics, economies, and societies, shaping the modern world dramatically. The conflict, often considered the deadliest in human history, left lasting legacies and influenced international relations for decades.