My vote for most valuable insight applying as much to natural fitness as to economic behavior it is this:

The most important part of the environment is the humans and what they are doing. If I and my merry band of 100 or 1000 or even 1000000 or even 1000000000 tribe members are contemplating how we should supply ourselves with food, shelter, weapons, entertainment, & c., we should first, foremost, and with great care look to use what is already developed, invented, and produced by the rest of the world. You were concerned about warlords having trouble extracting or refining oil, but you stumbled upon the reasonable assumption that obviously the Toyotas are going to come from Japan and don't need to be produced by the warlords.

Even in the US, about the single most effective source of new cool stuff yet to grace the surface of the earth, we drive Toyotas. And BMW, Mercedes, Fiat, Volvo, Hyundai etc. We get wine, cheese, movies, etc. from everywhere else. In some self-fulfilling sense, we import about as much as we export.

Could we go it alone? Sure. We'd probably be about 90% poorer. You can quibble over whether we'd only be 20% poorer or 95% poorer, but if you at all immerse yourself in a study of where stuff comes from, the expense of inventing vs copying, the benefits of mass production and massive specialization, you will absolutely unavoidably get the sign of the effect right.