The US, the most well-funded army in the world for the past generation, hasn't won a war since the end of WW2. Army size and the like does not matter. The Ukranian forces can entrench themselves and maintain a guerilla war for a long time, as long as they have supplies.
I strongly feel like wars are unwinnable. It either ends up in a stalemate and attrition - see the US trying to do a thing in the middle east for two decades only for things to go back the second they leave - or MAD. And the last one hasn't and will hopefully never happen.