You would think if there were a situation where the Union was united against a common enemy and a member joined that enemy, there would be recourse, but as it stands now the European Union doesn't have a mechanism to remove members that might go against it and neither does NATO for that matter. Could a country like Slovakia or Hungary share NATO tech and plans with an enemy?
Mainly because nobody every figured on member states doing something so monumentally stupid. But they're learning, and I fully expect that there will be a playbook for this sort of thing. You could see the exact same thing with NATO member Turkey suddenly playing both sides against the middle with respect to Finland (for no reason) and Sweden (ostensibly for the way Sweden has refused to join Erdogan's side in his internal politics) joining NATO. These agreements were made in a time when people were a bit more sensible so now adjustments will have to be made, or an entirely new agreement should be drafted. Greece and Turkey both in NATO has always been problematic as well but NATO needs Turkey so for the longest time there was a blind eye turned to the fact that officially two NATO members are still at war.
The assumption that NATO membership would cool things down a bit was imho mistaken, they should have picked either Greece, Turkey or none, but not both.
Poland, Hungary and now Slovakia all have burned a ton of credit by pulling these stunts and may end up really regretting them. But any country in the East out of NATO or the Union would be a massive boon to the Russians so that likely will not happen but there are plenty of ways to put on pressure without letting it get that far.
