Unfortunately, it comes down to the will of the people and the tolerance for casualties.

People forget Ukraine is not ethnically homogenous. They have a substantial population, mostly in the east, but also across the entire country that identifies as ethnically Russian.

If the Russians route early in the invasion, especially if they commit to amphibious landings in the south near Odesa, then I can see the Ukrainian government suing for peace within a week or two.

And this isn't some pro-Russia stance. Fuck Russia, but that is the unfortunate reality of the situation.