Starvation was highly scaleable tactic the Nazis used. Just pile people up and let them starve and also freeze to death (another scaleable tactic).

Food shortages could cause this but it would also cause riots/conflict before anything. Lack of heating could also do it. It won’t be an extinction level event, just a culling.

I foresee that before all else. It won’t take much, just sudden infrastructure shocks.