US President Joe Biden has pledged to aid Ukraine's pursuit of victory "for as long as it takes," without defining what the end state might be. Russian President Vladimir Putin has interpreted U.S. intentions to mean a fight "to the last Ukrainian."
Anyone with a discernible pulse is aware of the danger that the conflict could escalate into a conflagration large and destructive enough to morph into World War III. The threshold would likely be crossed once nuclear weapons were unleashed. The military doctrines of all nuclear powers stipulate that such an attack would justify an in-kind response, though without always ruling out the same for lesser provocations of a potentially existential nature.