The Expansion was a period of great upheaval, but ultimately led to the blossoming of humanity. After thousands of years trapped within Kas, the extranormal event which destroyed the barrier finally permitted humans to return to the surface lands where we first evolved and had lived on.
The social confusion during the time, as many people sought to spread out into many different parts of the globe resulted in misunderstandings with the limited number of people not trapped within Kas, called the Chidtty, which some of their descendants today still hold grudges over. As unfortunate as such issues may have been, nobody can deny that the world humanity has built since has been well worth the cost.
To quote the late Supreme Macch Kyolr:
Atlantis is the greatest political project in the history of mankind. We have built cultural towers, police search-spires, universities, and tamed many wretched wildernesses. You can stand in the city center of any part in our great nation and think back. You can turn to your neighbor and say, "you know, all of this land was empty when we first came here. We did this."