Edit: really don’t want to sound ignorant here, just that while growing up and traveling around Central Europe there was a strong belief that we‘re all in the same boat and I even remember maps showing all countries (except Switzerland) in shades of „blue“ but … I never looked more deeply into that - until now
Being part of NATO make it automatic. But Sweden would just have to ask for help.
At first news of invasion, the USA, the UK, France (e.g. 3 nuclear powers) and most other EU countries will start prepare, awaiting for the Sweden to call.
Fighters would be up in the air, patrolling along Sweden territory, ready to act immediately.
Anyway, that won't happen soon. The Ukraine war is far from over ; it's likely to turn into a guerilla with money and weapons provided by the West. Ukrainians will come and be trained to guerilla warfare by the Eu countries and Russia will bleed.
As large as the Russian army is, it can't control a rebellious country of 50M people. Some Ukrainians will join the Russians for sure, and most of the population will turn a blind eye, enabling the guerilla. The more the Russians react by punishing civilians trough indiscriminate shootings or mailings, the more recruits will join the guerrilla.
This is going to be bloody for both sides. Ukrainians have tasted enough democracy and freedom for long enough to go back to an authoritian regime like Putin's or a dictatorship like Belarus.
This could cause Putin to turn into an overt dictator in its own country. It will work for a while. And then he'll fall. That will be painful for all the peoples concerned: Ukraine, Belarus, Russia.
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