You have to defeat the enemy army in order to claim victory.
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Lots have nations have managed to still field armies in the past, and yet have lost. In fact I can think of two cases where a superior army still remained and yet that side LOST. In both cases they were US forces: Vietnam and Afghanistan. In a democracy the army is an extension of the political will of the people. Even Sun Tzu knew (millennia ago) that defeating the enemy army was THE WORST way to win a war.
The basic questions for this war are:
1) How much of Ukrainian territory does Russia need to control to force Ukraine to surrender? How large a force and how many casualties would it take to get there.
vs.
2) How high a level of conscription and how many casualties can Russia sustain without the people overthrowing the Putin regime.
True, you can degrade the enemy’s will to fight, but I didn’t mention that, since it’s not going to happen in a situation where you have placed the invaded country in existential peril. So the Russians screwed themselves on that count as well.