A chill ran down my spine when I heard this poem in Bryan Cranston's voice. I observed a transition of his voice from good to evil when he says "My name is Ozymandias, king of kings" and the cold ending just gave me chills.
Apparently, All kings and empires fall no matter how mighty they are.
Short answer would be, as Carlos Rui Ribeiro said "We really don't get know what happened".
I feel this is late answer for the question (judging from its Latest activity) but still I am going to write my interpretation of the movie as some part of my answer is not in existing ones.... (more)
Heisenberg is out for a drive when he's stopped by a traffic cop. The cop says "Do you know how fast you were going?"
Heisenberg says "No, but I know where I am."