It could be Russia's Viet Nam, in that they will be fighting a nation of resistors , and those resistors will be getting help from the US and the rest of NATO, just as the Viet Cong were getting help from the Chinese and the Soviets.

Imagine if a foreign power were to do the same to us. Imagine if Canada had once considered the US part of it and now wants us back. There is no way it could happen. Ukraine has had a couple decades to enjoy NOT being under the Soviet thumb, so to speak.

The Russians can not win this if there are enough Ukrainians to resist. Not to mention that Russia, as big as it is, has an economy half the GDP of California, for fucks sake. They can't afford a protracted war with an army (or population) that can't be defeated.

I don't know who said it, but it will apply to this at some point;

"The end of a war is not defined by the victor, but by the vanquished."

In other words, if a population refuses to give up and surrender, it doesn't matter who they are fighting.