Brief summary:

Eorzea was gearing up for war between the city states, then the garleans showed up and plowed down a city, then all the other eorzeans city states decided to band together to fight the garleans and then they....never showed up.. (Maybe they ran into "HOLY CRAP THAT IS A LOT OF DRAGONS"). Because the city states had gathered so many soldiers and it was apparent that they were no longer going to be going to war they started up the adventurer's guilds to keep the economy from tanking.

nifty job of explaining why we have so many adventurers.