Only many dirty nukes could contaminate enough land to create a significant food supply problem. Conventional explosives with radioactive material cannot contaminate a large area.
I'm sure there would be some level of disorder, but the U.S. would not fall apart even if there were a nuke detonated in a U.S. city.
Hmmm. I don't think that's true. Here in Great Britain a few years back we had a fuel protest. Basically, a handful of farmers wouldn't allow the fuel trucks to leave the refineries (or where ever they were leaving from). The gov't allowed the protest. Within days the supermarkets already had empty shelves of bread and milk. A couple days later, there was no fuel except for essential vehicles (ambulances, the cars of doctors/nurses etc). If that had gone on for even another week, it would have been pandemonium here. And all that without a nuke. There was nothing wrong with the food here- it just wasn't getting to where it needed to get.
All it takes is a vital link being severed. New York City cannot be fed without fuel. Without fuel, the people of New York City could also not flee far enough to get to food (assuming they could pay for it once they got there). The areas where they would flee are not set up to handle millions of refugees. Our whole system relies upon so many links working effectively. Any of these links breaking has catastrophic effects. If the terrorists could somehow sever the command structure from the nation temporarily and get a chain reaction of events going- it would have a high likelihood of spiralling out of control.
I have seen the effects of this also in Africa. Mozambique for example. Torn by years and years of civil war. The local game hunted out. The people had no way left to really sustain themselves. The second largest city- Beira- was essentially isolated from the capital because the main highway down the coast had become inpassable and also because of the lack of fuel stations along the way (look at the map- it's fairly far). Beira was left to slip into decay. It was really spooky moving around those old colonial streets. The people there were afraid. They'd hunker back in the houses and watch you walk down the sidewalk. Big ships were washed up on the beach and rusting because the system of looking after them had broken down and the sailors eventually had to abandon them. It was like the twilight zone. Hundreds of kilometers to the south, the capital, Maputo, was a vibrant, thriving place. And the war was over. Nobody had managed to connect the country back up to that point though- although signs were looking positive.
The point being, events have a way of taking on a life of their own and getting beyond the ability of anyone to control.