Nobody has the force projection necessary to invade the US, nobody even builds their military with that goal in mind.
WWIII doesn’t have to involve Russians going into your house for the US not to be unaffected, while we are a net food exporter, the majority of that is animal feed. We import most of the food you see in grocery stores, tons of parts for our power grid, refineries, and other vital infrastructure is imported too. The Yemen is managing to shut down US-aligned shipping off their coast with drones and missiles, despite getting indiscriminately bombed with US bombs and planes for a decade.
Occupations suck for the guys getting occupied. Iraq allowed 1 machine gun per family. Do you think the Iraqis had a good time when we occupied them?