Yes, the "NATO problem". The "problem" that is actually defending countries from invasions instead of letting their people, in a country that is not even close to being in NATO, die at the hands of tyrants.
Because Russia just took Crimea a few years ago and NATO refused to allow them to join. And Russia has agreed Crimea was part of Ukraine as recently as 1994.
Again. Reason is something one has in this mind. You may find no reason where a different person sees one.
No need to justify any of that, because it all sucks and is ultimately utterly pointless. It does help so, to understand the other side. Because it makes it easier to find a solution that doesn't lead to all out war between nuclear powers. And no, I don't consider just handing Ukraine to Putin to be that solution. Or Taiwan to China, as far as that is concerned.