"And thats when the bombs started dropping..." - Kyle explaining the Nuclear war
The Nuclear war started when the Government destroyed New York city with a nuke in 2032, when The virus was unleashed across the city. However, the destruction of New York did not stop the virus as it spread across the world. Many more cities and locations were destroyed along with most of the land.
When the war was over, and the world's governments collapsed, the only order left was Johnson's ruthless Company. The Nuclear war marked the end of the old world and the rise of Johnson and his Company.