World War II officially began on September 1, 1939, when Germany invaded Poland. This invasion prompted Britain and France to declare war on Germany, marking the start of a global conflict that would involve numerous countries and lead to significant geopolitical changes. The war arose from unresolved issues following World War I, including economic instability and the rise of totalitarian regimes in Europe. The conflict would eventually expand to include much of the world and last until 1945, culminating in the defeat of the Axis powers and the establishment of the United Nations.