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Winds blow east in that part of the world.
They were counting on US support so they played nice. If Europe fails to do anything meaningful we might see UKR do something incalculable.
UKR knows that the only way to stop Putin is to match his posture. They have openly stated last year that they should get a nuke since US is unreliable.
Retaliate for nuking their own territory?
Cut off a limb to save the body. Radiation contamination is a pretty effective ground denial strategy- make those eastern oblasts uninhabitable for a 100years. That would afford some time for Russia to turn itself inside out.
I’m not suggesting it’s any planners first choice, but it would certainly protect the border from further ground invasions.