As is usually the case, war will occur because one side fundamentally misunderstands the nature of reality. Putin, as Tzar for life, has a cold war view of the world, a zero-sum mentality. As he and his thuggish cronies have eviscerated the Russian economy (which now relies on exports of oil & gas almost entirely to prop up the collapsed ruble) he has millions of younger Russians looking Westward. His only "solution" is to make the West look as unappealing as his own kleptomaniac regime, and as Ukraine, despite endemic corruption and shoddy infrastructure, has been doing better than Russia thanks to its still-slender ties to the EU, the comparison is unfavorable. So destroying Ukraine's future is essential in order for Putin to execute his sad little vision of ever-worse-but-I-cling-to-power-regardless.
The West, meanwhile, is collapsing as representative democracy inevitably means populism and populism inevitably means ever-more-grotesque stupidity. And so, just as in the 1930s, the West will sit on its hands and do nothing of substance.
In a less stupid world this scenario would not arise; in a fractionally less stupid world, the West would do what Russia always does: claim that "volunteers" are going into Ukraine in a manner that has absolutely nothing whatsoever to do with official government policy, and all the attack helicopters and tanks and other materiel are merely items brought along by patriotic volunteers - just as has been the case in Krim and Donbass. This would force Putin to understand that invasion will result in real conflict, not the easy crushing of antiquated Ukrainian forces. But such a stand would require the one thing the West does not have after nearly 80 years of peace: a spinal column.