Another clue may been in the Mayor's speech at the end when he said the phrase "against all enemies." I wasn't in the military but isn't part of the military oath, "to defend the United States, against all enemies, foreign or domestic ?
Also, the nuclear explosions may have been used to confuse the public and allow the military time to ...
1.) Secure power generating plants. This would allow them to cut off the power (and after an attack, who would think anything nefarious about the military "protecting" the power plants?)
2.) Secure communications, take TV/Radio off the air. Take over Telephone Central Offices (cutting off voice and data transmissions)
I think this reenforces the idea that only certain cities were hit. If you hit Washington, New York, LA, and Atlanta...you basically take out leadership, the major broadcast networks, and the major news networks. I still have no idea what is Denver worth striking. Maybe a local can help.