Russia has a regime that’s clearly hostile to Ukraine. Additionally, they have a history of trying to assisted people in their way politically. So, if you’re successfully pushing back the army, you have the momentum to try to overthrow this regime and ensure some long term security on your border. Whether or not that would work or be wise would be separate calculus but nuclear weapons in the hands of an unstable leader adds at least a little bit uncertainty that would make you pause in addition to everything else.
> why would they do that
Russia has a regime that’s clearly hostile to Ukraine. Additionally, they have a history of trying to assisted people in their way politically. So, if you’re successfully pushing back the army, you have the momentum to try to overthrow this regime and ensure some long term security on your border. Whether or not that would work or be wise would be separate calculus but nuclear weapons in the hands of an unstable leader adds at least a little bit uncertainty that would make you pause in addition to everything else.