Since I can't find the video where Louisoix explains everything about 1.0, here's a video from Mr. Happy:

http://www.youtube.com/watch?v=b6G7LyLv1cg

The event where Bahamut comes down and reeks havoc was the final cutscene of 1.0 when the servers were shut down. The events of 2.0 take place directly after, the heroes of 1.0 forgotten - only known now as the Heroes of Light.