Seriously? Those ancient Brits were probably happy to see the Romans leave given that they could quit paying tribute out of their limited resources. They had already mastered the technology, farming, husbandry, and wild food resources to scratch out a living and had a relatively low population density. Americans are entirely dependent on modern tech support for food, energy, shelter, medicine, and have no experience working together in the size of cooperative group necessary to provide the needs of daily life with primitive tech, let alone how to employ that kind of technology. Preppers live in a fantasy world.