World War 2, also known as the Second World War, was a war fought from 1939 to 1945 in Europe and, during much of the 1930s and 1940s, in Asia.
The war in Europe began in earnest on September 1, 1939 with the invasion of Poland by Nazi Germany, and concluded on September 2, 1945, with the official surrender of the last Axis nation, Japan. However, in Asia the war began earlier with Japanese interventions in China, and in Europe, the war ended earlier with the unconditional surrender of Germany on May 8, 1945.
The conflict spilled over into Africa, included a handful of incidents in the Americas, and a series of major naval battles.
It was the largest armed conflict in history, spanning the entire world and involving more countries than any other war, as well as introducing powerful new weapons, culminating in the first use of nuclear weapons. However, despite the name, not all countries of the world were involved; some through neutrality (such as the Éire - though Éire supplied some important secret information to the Allies; D-Day's date was decided on the basis of incoming Atlantic weather information supplied from Ireland - Sweden, and Switzerland), others through strategic insignificance (Mexico).
The war ravaged civilians more severely than any previous conflict and served as a backdrop for genocidal killings by Nazi Germany as well as several other mass slaughters of civilians which, although not technically genocide, were significant.
These included the massacre of millions of Chinese and Korean nationals by Japan, internal mass killings in the Soviet Union, and the bombing of civilian targets in German and Japanese cities by the Allies. In total, World War II produced about 50 million deaths, more than any other war to date.