Putin wants to rebuild Russia as it was in 1989. Ruling everywhere from the Baltic to the Pacific. Lands that were given their independence in that year.
He started small supporting rebellions in remote areas of Georgia, and Moldavia, recently he has supported the government in Belorus. It is now a client state. he has done all this with one eye on NATO, to see how it has reacted - and, he saw that NATO was doing nothing.
Then in 2014, he invaded the Crimea and supported pro-russian groups in the Donbas region. Once again he had got away with it. We faced a future of Putin gradually re-occupying and recreating the old Soviet Union and creating another military state. That is why NATO had to say, thus far and no further.
Remember the poem by Martin Niemoller?
First they came for the Communists
And I did not speak out
Because I was not a Communist
Then they came for the Socialists
And I did not speak out
Because I was not a Socialist
Then they came for the trade unionists
And I did not speak out
Because I was not a trade unionist
Then they came for the Jews
And I did not speak out
Because I was not a Jew
Then they came for me
And there was no one left
To speak out for me
Replace communist, socialist, jew, with Ukraine, Moldavia, Poland, Romania etc.
That is what will happen if we do not support Ukraine.