>WTF happened??
Peace and wealth happened and, like every single great empire before, the lack of competition and lack of threat created depravity and an "ah well" attitude based on arrogance that "nothing can happen".
Peace and the feeling of being invulnerable have shown, time and time again, to weaken great empires and cause the people within them to seek out new reasons to support often those being adoption of those they previously harmed into their inner ranks which then topples the system eventually with so many competing systems in play.