The nuclear weapons in Ukraine question was settled in Budapest in 1994, when in exchange for surrendering physical control of the third largest nuclear arsenal in the world and acceding to the non-proliferation treaty Russia , the UK, and the US commited to respect the independence, sovereignty and existing borders of Ukraine; confirmed their obligation to refrain from the threat or use of force, or economic coercion to subordinate Ukraine's exercise of sovereign rights to their own interest.
By his reaction to the Euromaidan including fostering of separatism in Yanukovych supporting Oblasts and occupation of Ukrainian territory in Crimea in 2014 Putin tore up the agreement. Prolonging violient separatism was a way of indefinitely postponing the chance of NATO accepting Ukraine's wish to join.
I do wonder if his plan is to create the previously threatened South-East Ukrainian Autonomous Republic as that's where he is having success and appears to be using better forces and that the action in northern central Ukraine is a brutal diversion and he declare peace and withdraw from those areas once he has consolidated his puppet state in the relevant oblasts and separating the Western leaning ones from the sea.