If a group or groups threatened trucking with road side bombs/attacks and impacted the ability of food to get into grocery stores along with other avenues of attack (power, gas, etc) things would destabilize pretty quickly with riots/looting. You can shrug it off as third world country problems if you want but that is likely how it would start. Smaller groups being emboldened into that type of action, then other groups following their lead, then other groups deciding to join in after the government starts blowing up American citizens, and the spiral continues. Then eventually you start seeing the larger militias, warlords, etc rallying people to their cause for civil war with other groups/the remainder of the government. This is not a new third world idea - look at the rise of the Nazis for parallels to the rise of ISIS. Chaos is sewn, partially by them and by others and then they start rising/holding power in areas.
“There are only nine meals between mankind and anarchy.” - Alfred Henry Lewis