Ghostwalker did not come through the Watergate rift alone, the reports said he lead an army of spirits.
Dropping a nuke on Denver is not really an option, the CAS, PCC and Souix Nation will not be happy with you. Then if the attack works you will have the other Great Dragons ready to make an example of the UCAS. The good news is if it fails you'll only have one really pissed Great Dragon.