When the world was twice engulfed in great wars in the 20th century, the causes lay not in a single event, but in an accumulation of political, economic, and military tensions built over years. World War I and II left massive devastation, and behind them, the United States played a pivotal role, both in the final stages and in shaping the postwar order.Roots of World War IBy 1914, Europe was trapped in a complex alliance system. The Triple Entente—comprising Britain, France, and Russia—faced ...