The major global conflict that began in 1939 was World War II. The war was ignited by Germany's invasion of Poland on September 1, 1939, which prompted Britain and France to declare war on Germany. It quickly expanded beyond Europe, engulfing many nations worldwide as alliances formed between the Allies and Axis powers. The conflict was characterized by significant battles, atrocities, and large-scale mobilization of troops and resources. World War II had lasting implications, reshaping global politics, economies, and societies, and leading to the establishment of the United Nations to promote peace and prevent future conflicts.