The Ukrainian war has been presented to me as a mad man trying to take over the world a la Hitler. I think it's more complicated and concerns about NATO expansion, the US Dollar as the world reserve currency, and Russia controlling warm water naval access make sense as motivations for the war, even though they are also be tools of propaganda. It seems clear that Russia believed they had the opportunity to establish themselves as a great power once again alongside China and the US in the "new multipolar world" they harp about.
My question is this: In light of this information, why has the Ukrainian conflict become seemingly (based on resources allocated and increasingly provocative drone incursions into NATO territories) existential for Russia? Are the us sanctions crippling long term without Ukrankan resources? Why are they willing to sacrifice so much if they already have Crimea free and clear?