War between the EU and US, however unlikely, probably wouldn't mean all your devices stop working right away. At absolute worst, patches would stop rolling out to EU-owned and operated devices, so you'd still have your hardware, the current version of your OS, your local files, etc.

Back up all of your iCloud(it can be exported). Keep a copy on hand, and keep a copy encrypted in the cloud using something like BackBlaze.

Even though Linux has strong ties to the US, some of the distros aren't wouldn't be completely restricted (I'd wager most of them), and aren't all fully US-based. Keep a copy of Asahi / whatever flavor can run on your Mac handy on a bootable USB stick.

You could grab a cheap android phone as a backup to your current iPhone and flash it with a fork, like Graphene.

The general data protection and backup strategies largely apply and work well for this scenario.