The Purge (Attack Machines)

The Purge was an event that hit the world on March 1/2 (depending on the timezone), 1946. It destroyed most of human civilization, to people outside of cells. The world's population decreased from roughly 2.5 billion to just 145.000.

History
Following the discovery of the mysterious Kraventhorn during WW2, a mysterious plan emerged saying that members of the organization - of unknown source - said that they wanted to kill all humans. Once the war ended in June 1945, there was hope that Kraventhorn wouldn't commit such acts. However, intercepted radio transmissions made by Kraventhorn in July had led to the creation of cells worldwide. There were shortcomings, though, as these cells were limited.

In the middle of February, Kraventhorn did the impossible and released the bombs. They weren't bombs but simple repellents to kill humans and some animals. Over 145.000 survivors were held in cells for days. After an undisclosed period of time, the cells were opened and human resettlement had started.