This explanation doesn't make much sense. If Ukraine maybe joining NATO in an unspecified future is such a big threat to Russia, then surely Finland joining it right now is an even bigger threat. Not to mention that Poland and the Baltic states already are NATO members.

Russia's security is guaranteed by nukes. Security definitely wasn't the reason why Russia has invaded Ukraine.

If security is guaranteed by nukes, why does the US threaten the Salomon Island? Placing missile defense systems close to the launch sites gives you the possibility to intercept them early in the boost phase when they are still slow. Having assets close to enemy borders in general gives you shorter reaction times. This is about tipping the scale somewhat in your favor, not ensuring your victory.

I am not qualified to judge the situation with Finnland, but not all neighbors are necessarily equal. Things like geography, for example, might play a role and Finnland in, Ukraine out might be more desirable than Ukraine in, Finnland out.

Russia is busy with Ukraine so can't deal with Finland however they have been talking tough and posturing.

NATO members are NATO members, attacking them after they've joined NATO an orginisation basically created to fight Russia would mean you go to war with all NATO members.

Russia's preference methodogly of defence is creating distance. With Ukraine, NATO would have been extremely close to Moscow. Finland is further away but they're still not happy about it.

NATO expanding into that region was well known to be something that would threaten Russia, since it was founded to fight Russia, to the point Joe Biden said decades ago[1], that Russia expanding into that region would cause conflicts.

[1] https://multipolarista.com/2022/03/08/biden-nato-expansion-r...