I think some people have the impression that the issue is isolated to Ukraine and it's sovereignty. It is not.
We have had around 80 years of a mostly peaceful Europe, and that means most people from recent generations think that peace is the norm and that war is consigned to the history books, or at least, there's no realistic threat of it.
One of the major reasons for that peace had been the existence of NATO and, paeticularly, US involvement. Russia has the second strongest military in the world and is a nuclear superpower...but Europe had the US as an ally, which deterred aggression.
That has all now changed.
It isn't just that the US are pulling support, they are becoming actively antagonistic towards Europe. They are advancing Putin's aims (while brazenly peddling his propaganda), extorting Ukraine, interfering in elections and threatening to invade or otherwise annex part of Denmark.
We've gone from being protected by one military superpower, against another, to AT BEST having one enemy and one who is ambivalent...or, at worst, both as being enemies.
A failure to invest in defence spending, at this juncture, is an invitation to a war (which may well have a nuclear dimension) that Europe will lose.