It’s one of those symbolic initiatives. There may be an official mechanism but right now, it would be a disaster without NATO. Right now, the US has most of the Command and Control logistics (think constant satellite connection, missiled detection systems etc.) That stuff is super expensive and the assumption was that America was an ally, so not a lot of duplication was built in.

A NATO without the US dooms Ukraine and presumably, whatever hits of Eastern Europe Putin feels like holding.

It’s shitty, frustrating and awful but it’s also the grim, current reality. We didn’t realize our allies would become two bit thugs.