One thing that worries me about this war is the parallels I see with a couple of other wars - the Soviet invasion of Finland in 1939, and the recent Sri Lankan civil war of 1983-2009.
Finland: a country that had been part of the Russian empire until 20 years previously; a manufactured excuse to attack Finland; a poorly armed but highly motivated defender; a large invading army with modern military technology expecting a walkover victory, and being brought to an unexpected halt; and final victory only being achieved after a prolonged halt to regroup, followed by a second attack supported by massive firepower.
Obviously there are differences, given that the USSR wasn't interested in conquering Finland, merely pushing its border with Finland away from Leningrad. But it points to a way the current war might progress.
Sri Lanka: both sides saw themselves as a threatened minority - the Tamils as a minority within Sri Lanka, and the Sinhalese as a minority when compared with Tamils in Sri and India.
The comparison here is that the Ukrainians see themselves as the obviously weaker party compared to Russia, but the Russians also see themselves as threatened and "surrounded" by NATO.
The danger here is that it lends both sides the desperation generated by the belief that defeat equals extinction, meaning that neither side is motivated to negotiate a peace deal which involves much in the way of concessions.
What concerns me is that Ukraine is likely to want peace to involve a return to pre-war borders, including the breakaway regions in eastern Ukraine. Given that the people there almost universally want to be part of Russia, it's hard to see how they could ever be reconciled to life as part of Ukraine; on the other hand it would set a dangerous precedent to allow Russia to emerge from the war with additional territory.