I think so-called suitcase nukes are more designed for tactical use and not for city destruction.
They would create havoc and large scale death, and if exploded at ground level destroy square blocks of a crowded city. They will make a big mess and kill many thousands.
It would trigger a worldwide economic disruption, and if the terrorists were extremely lucky, decapitate the leadership of some of the victim nations involved.
That said, I think the US & NATO & Russia absolutely go crazy against Islam terror, N Korea, Iraq and Iran, force India & Pak to disarm, round up many 10s of thousands moslem immigrants, install much higher levels of security on maritime trade, air traffic, border controls, and monitoring of telcomm.
The age of easy migration around the world will end.
The reactionary islam behind this will be destroyed in the end by moslems who despise it and the corrupt politics it enables.
The world would go on, in the end but never the same.
Ed