First off, the only 5 mega ton nukes in existense are ICBM that would have to be launched from somewhere. they are the Soviet Sapwood, and the US Atlas.
25-30 meters long and 250,000 lbs - no one is smuggling one anywhere
That being said, the country the launch came from would cease to exist, the only possible answer would be 100% nuclear retaliation.
The most damage that could be done by a suitcase nuke would be a little more than the damage done on Sept 11, the real damage would be the panic about a nuke being detonated on our soil. people would be hoarding duct tape and building bomb shelters.