5 A morally sound populace with the goodwill to cooperate with each other and a commitment to make it work.
Cripplecreek is right, if you cut off the food from L.A. it would look like a Snake Pliskin movie in a week...but consider that the people who make the food would never want to starve a city. That's why famine is a way of life in Africa and pretty much unknown in the West.
The cities would fast become a source of labor for growing crops. It's not a big deal for us to grow food for ourselves but if the cities need food they would have to supply huge ammounts of labor.