The suitcase nukes are already here, we had a top level Russian defector tell us this twenty years ago. They will take out the entire power grid in these cities and they will take out a few city blocks and they will cause terror, panic and confusion. Communications go down and if they are coordinated hitting NYC, Washington DC, Atlanta, LA, etc all at the same time no one would know what is going on. The way the power grid works you take out ten of these major transformers around the US and the entire grid nationwide goes down. At that point, in the confusion, all those people who are already in the country could cause havoc, cutting power lines, causing damage to transformers, and critical infrastructure so that when they try to bring it back up you continue to discover problems crashing the system.
The torpedo nukes cause giant radioactive tsunamis that would cause massive destruction to our coastlines as much as 5 miles inland in places. That is where 65% of the US population lives.
All of this could hit at the same instant.
Yes the military has back up generators and yes, not all our military bases are near the coastline, but every person in the country would be greatly affected and how much diesel do they have stored up at the military bases for those generators? Without power you would soon have a crippling shortage of diesel and without that how do you maintain order in cities across the country that are blacked out. Looting and gang violence would be everywhere.