Welcome!
As to "How did it happen?", well I'll give you the short version
We go under the premise that the world becomes more and more isolationist. Trade buriers are put up between friendly nations, Stock Markets around the world crash, a Global Recession takes place... you know... what's going on now!
Anyways, these pressures build and due to regional events, countries begin to fragment. This gets worse and worse until virtually every country has dissolved into splintered economic and military sub-regions. The desire is to re-unite the nations and with that in mind, you set out on your quest to become Supreme Ruler.
Different regions have different personalities that will give each scenario a unique feel... Western Democratic Regions, for instance are particularly interested in ending the tensions and opposed to military conflict. A tendency in Western Scenarios will be to have scheduled re-unification votes, wherein the winner will go on to the next round...
In other regions of the world you have events such as the re-birth of Communist Russia, an unchecked Dictator in the Middle East, the religious conflict of India and Pakistan, North Korea's desire to take the South, China's eyes on Taiwan, and much more...
How you play each scenario will be up to you... Militarily, Economically, Politically, etc...