We care about Ukraine because we don't want Russian domination in eastern Europe-- it's simply a matter of security.
Obviously security must go first, but if we can afford it morality is often good long-term.
Why can so many of you not see [beyond simplistic 'goodies' and 'baddies' labels] how much Russia feels threatened by that? Imagine if the cold war had been 'won' by the other side; Canada and Mexico had joined the Warsaw Pact and the USSR had troops and ICBMs stationed there. And now Alaska, which had declared independence from the rest of the USA was talking about doing likewise [0] How do you think the US would react?
Clue: look up 'Cuban Missile Crisis' if you need a hint.
One of the fundamental pre-requisites for solving crises diplomatically before the shooting starts is seeing things from the other guy's point of view. Simply dismissing 'them' as the 'baddies' and discounting their view of the world only leads to more conflict --as has happened in Ukraine.
[0] Baggsy a share of the royalties, if someone decides to write this up as an Alternative History blockbuster!
A bunch of European countries joined. They weren't forced to.
Of course the Russians feel threatened, but that doesn't justify anything. It's possible I too would have come to the conclusion and invaded Ukraine, if I were in their place, but it's still against the UN charter and it's still a war of aggression.
Countries don't have a right not to have huge alliances on their doorstep. That doesn't mean that they can allow them, so perhaps the war was inevitable, but the Russians don't have the power to take what they want anyway without losing more than they can afford to, so a necessity argument becomes dubious, and even a correct necessity argument wouldn't be a justification, because it would still be a war of aggression and against the UN charter.
You could also flip it around, and see it from a Ukrainian point of view, then there's a country that demands that they not join NATO, but imagine receiving such a demand? It goes beyond what is acceptable in the international order as we see it, a demand that another state subordinates itself.
I remember something someone in Sweden said, that if Russia were allowed to have their buffer state, Russia would force a large population than that which Russia itself has, to follow Russian policy in who they get to ally with. It's a comical thing. It's not quite true though, depending how you count, but even Ukraine alone is 40 million people, that alone is demanding to control another country with a population of 20% of Russia itself, so while it might not be right mathematically the point is still pretty sharp, I think. Imagine if Sweden demanded that Norway do as it wants, because it has smaller population.
The reality is that Estonia demanding a buffer between itself and Russia is no less legitimate than any demand of Russia to have a buffer between itself and NATO, which is to say, not legitimate at all. Peoples get to rule themselves and their historical lands, not anything else.