Ukraine is technically in Europe but so easternized it may as well not be. Culture is all.
The US etc have no strategic reason to specifically declare a strict limit because that is bad strategy. Strategic ambiguity is vital because when outcomes are known instead of guessed the side with the initiative (Russia) can route around obstacles. NATO/cultural "Europe"/EU lacks credible strategic ambiguity.
NATOs real strategic options are not ambiguous.
Putin knows:
NATO cannot risk escalation.
NATO does not have sufficient power projection and geography matters.
NATO does not have time to build effective Ukrainian deterrence.
NATO does not have popular support for war.
NATO relies on US logistics (logistics are everything) diverted elsewhere and units not stationed in nor quickly able to deploy that far away.
NATO was never configured for deep operations far from the English Channel.
NATO in 2022 is a shambling distracted ghost of 1990 NATO.
Shock assaults with enough momentum might capture Kiev even with ongoing light local resistance. (Russia's dealt with such before, see Ukrainian post-WWII resitance: https://en.wikipedia.org/wiki/Ukrainian_Insurgent_Army ).
Kyiv need no be fully secured to be out of the game early and could be cut off long before it's captured. Russia could cut off eastern and western Kyiv by destroying the bridges and any ferries, pontoon bridges etc it can strike. Each half of Kyiv has its back to the river. Heavy forces won't be able to move between them so resupply of the eastern side will be problematic. There is no room to maneuver.
The US etc have no strategic reason to specifically declare a strict limit because that is bad strategy. Strategic ambiguity is vital because when outcomes are known instead of guessed the side with the initiative (Russia) can route around obstacles. NATO/cultural "Europe"/EU lacks credible strategic ambiguity.
NATOs real strategic options are not ambiguous.
Putin knows:
NATO cannot risk escalation.
NATO does not have sufficient power projection and geography matters.
NATO does not have time to build effective Ukrainian deterrence.
NATO does not have popular support for war.
NATO relies on US logistics (logistics are everything) diverted elsewhere and units not stationed in nor quickly able to deploy that far away.
NATO was never configured for deep operations far from the English Channel.
NATO in 2022 is a shambling distracted ghost of 1990 NATO.
Cutting off urban centers of gravity can reduce necessity for ground assault. Putin will have considered multiple options including taking sufficient territory to render Ukraine economically unviable.Assaulting Kiev would be a whole different category than encroaching in the east where all those Russian sympathizers live.
Shock assaults with enough momentum might capture Kiev even with ongoing light local resistance. (Russia's dealt with such before, see Ukrainian post-WWII resitance: https://en.wikipedia.org/wiki/Ukrainian_Insurgent_Army ).
Kyiv need no be fully secured to be out of the game early and could be cut off long before it's captured. Russia could cut off eastern and western Kyiv by destroying the bridges and any ferries, pontoon bridges etc it can strike. Each half of Kyiv has its back to the river. Heavy forces won't be able to move between them so resupply of the eastern side will be problematic. There is no room to maneuver.