Putin knowing he has been reduced to 1/4 of a Trump, knows his country on the global stage is nothing more than a gas station that pushes liquified gas and petro-products.

So threaten war get an oil price which allows the government to continue to run. If remember correctly from an Economist (circa 2016) I believe the floating rate was $70-$80 per barrel. So anything above that is bank to them, so Putin has taken climate change and is profiting off of it.

Secondly, food.

Russia needs to be able to feed it's citizens and given their closed loop economy, they need to source internally, and the Ukraine is fertile ground.

I appreciate 110% your history, I looked it up myself, and yeah so much back and forth going all the way back to the Ottoman Empire. I also believe Poland also controlled it at one point also.