Modern farming is indeed dependent on imports. But modern-"ish" farming, with tractors, combines, and some degree of mechanization can still be done and maintained without microprocessors, sensors, and precision pumps and machine tools.

And if your supply requirements decrease by 150MM people, that makes the job substantially easier to manage.

> I do not believe the people yelling "secession!" are prepared to revert to a pre-WWI world.

Ah, but they think they are. Which is really the only thing that matters, since beyond a certain point, it's too late.