The US military actually goes out of its way to source domestic chips where at all possible. Currently this means a lot of Intel (or the older AMD/IBM chips made at GlobalFoundries I suppose), and there is a pretty significant amount of trailing-edge fab capacity in the US still, but you're right that it's not everything and there's some stuff they have to source from other countries.

That is actually the consensus reason (not sure it was ever stated outright) for the original plan for the 20k wf/m fab TSMC was building in Arizona (that has since been upgraded to 100k). Even if it wasn't necessarily a huge plant in terms of global supply, it gave the US defense/intelligence community a guaranteed supply of chips with domestic manufacturing and chain of custody.

It is nuts just how much of cutting-edge production is concentrated in the pacific rim region there, and the current shortages have really driven that point home even harder. Even leaving aside Chinese invasion of Taiwan, there could be a tsunami or some other natural disaster that takes out TSMC and that would just absolutely gut the worldwide economy. Imagine the current shortages where everything is still running at capacity, and then imagine that TSMC or some other major fab would go offline (Samsung getting hit in Korea, etc), be destroyed and take years to come back online - there are the same limitations as bringing new capacity online, you have to wait years to source new machines from ASML and so on.

It potentially could happen anytime. It could happen tomorrow. Just another big earthquake that sends a tsunami at just the wrong place and compounds all the current shortages even worse. Even apart from the national defense aspect, it is a dangerous concentration of irreplaceable supply chain elements and I think the US, EU, etc are really pushing to mitigate that danger a bit.