We are talking about the Third World War, which took place in the late XXI century, caused by the sudden depletion of fossil fuels.
This meant the beginning of the so-called Energy War. During this war, which lasted many, many years, a huge amount of countries disappeared, and those that managed to survive, remained divided due to the different aims of the people who lived in them.
For this reason, now the world remains divided in different groups of people, such as Phoenix, Crow, Aegis, etc.
If you want to know the whole story, go to the Energy War section.