World War II was the defining disaster of the twentieth century for millions of people across the globe. It was the culmination of the vision of total war the world had first encountered in World War I, but it was generalized to vast stretches of the planet, not just parts of Europe, Africa, and the Middle East. The promise of technology was realized in its most perverse form as the energy of advanced industrialism was unleashed in weapons of mass slaughter. World War II was also the setting for the Holocaust, the first and only incidence of industrialized mass murder in world history.
The war resulted in approximately 55 - 60 million deaths, of which 25 million were Soviets and 6 million were the Jewish victims of the Holocaust. While nationalist rivalries and international tensions certainly led to the war in some ways, as they had in World War I, the primary cause of WWII was unquestionably Adolf Hitler's personal obsession with creating a vastly expanded German empire. Europe had, in some ways, stumbled into World War I. World War II was instead a war of aggression launched by a single belligerent, Germany, supported by its allies. (Note: Germany, Italy, Japan, and their allies are referred to as "The Axis" in World War II. Britain, the US, the USSR, and their allies are referred to as "The Allies" in World War II.)