I really want to belive this: That it's somehow possible to work around such a kind of digital meltdown when the US really, really want to cause some real damage using zerodays, backdoors and so on. I can't imagine that it's possible, but I also don't think it'll come this: There'll just be a call from Trump to ${SOME_HEAD_OF_STATE} stating that if they don't shut up regarding greenland at this very second, they're looking at deleting the entire nation's mail, data, have no iPhones, Androids and so on. No one will even consider taking such a kind of risk. And that's just one factor besides tariffs and everything else.
Now this would be an interesting war. Regarding germany, almost all admnistrations from the smallest tows up to the government are busy uploading their data to US-clouds. They've just introduced the new e-health system where roughly 75% of all data (including encprytion keys) are stored on IBM systems (hello CloudAct) and so on, so the US can already access probably most of the country's data in a perfectly legally fashion. There wouldn't ever be a war because the US can simply shut down all the US OSses (long-term in a "nice" fashion by stopping export for software / security patches, or short-term by introducing nasty code in the updates). In practice, I wouldn't be surprised if they could shut down a typical country in a matter of hours if they'd really mean it.