I think it takes two things to justify the declaration

of the war: 1) NATO eastward expansion. 2) The atrocities at the Russian descents by the Neo Nazis in Ukraine.

For Putin, it is hard to say which is more bitter, but from international ethics, 1) alone isn't sufficent to justify a war. Certainly, in a similar situation, the US or NATO would think otherwise, but they are not good examples.

The core west knows this, that is why their media always "fails" to report 2).

In fact, 2) alone would justify an invasion though not necessarily a war of this scale.

But I think, without 2), Putin sooner or later would still do something for Russia is just as brutal and selfish a country as the US is.