When we pressure Ukraine to give up its nukes that guaranteed Russia would eventually get around to dismembering or absorbing Ukraine. Nobody in Russia believed the security guarantees extende to Ukraine least of all the western countries making them. Everyone expected the Russian invasion would be complete in a week or two, but the Russian army proved to be monumentally incompetent and the Ukrainians army surprisingly resilient. At that point only did it become clear this would not be a quick war. Ukraine lacked air power and modern equipment. Russia lacked competence and was over reliant on artillery and a mode of warfare dependent on leveling to the ground ever square inch of territory in front of it. That style required a massive logistics tail that did not exist.
At this point the war is turning steadily against Russia. Massive mobilization won’t save them because they are out of equipment, ammo, trucks, roads, rails, and money. Ukraine will get F16s in the Spring, western munitions factories are gearing up and soon Russia will be outgunned, out generated, and out moraled.