Six months feels insufficient. You'd want several years, at minimum.

I think there are two ways to effectively mitigate this risk: 1) have mining and manufacturing of your own that covers most of your needs, or 2) balanced trade where you get something critical from another country, but they also get something critical from you (and can't easily get it somewhere else).

(Of course when you have very solid allies, you can relax a bit more and rely on them, but you still have to be prepared for a situation where that ally has a shortage and prioritizes their own use.)