The war will spread. It will not be contained.
Those attempting to respond to the "humanitarian crisis" will also be attacked by the Russians, and will require protection. Think Somalia.
The Russians will be driven to attack the supply routes for Ukranian resistance. Think US bombing of Cambodia and the Ho Chi Minh Trail.
If NATO members in fear for their own sovereignty, the governments may feel popular pressure to act against the Russians unilaterally.
If you live in Estonia, Lithuania, Latvia, Poland. Hungary, Romania, and Bulgaria, do you really trust that the Western powers will protect you?
There's a history of the West ceding central and eastern European nations to maintain a nominal peace. In the late 1930s, the West was perfectly willing to let Hitler have the Saarland, Austria, the Sudetenland, and the protectorate of Bohemia and Moravia.