Well, Actually, I think the BIGGEST failure is NATO.
They actually lost one of the best opportunity to confront and defeat Russia, at minimal cost.
By yesterday when RuAF cleared the skies of Ukraine and established air superiority over the region, NATO actually lost their best time to intervene. Right now, any workable effort by NATO (without starting WW3) won't only be "添油战术".
Had NATO rapid reaction forces reached Ukrainian front right after Putin started the operation, they would have a change of confronting the Russian forces by just staring them down without really starting a war. Or, they should have stepped in when Russian forces are still on their way to their initial position, anytime during the first 3 days of battle. That would also result in a stare down with the Russian troops.
In fact, what you are saying, (that the West knew what Russia planned to do and yet didn't act) will simple confirm the fact that strategically, they lost their best chance to humiliate and deter Russia for decades.
Haha do you have any idea about military movements and war?
Nato doesn't even have more than 10 thousand force that they can bring to ukraine quickly. Even that so called rapid reaction force is miniscule compared to the force arrayed by Russia which 200 thousand plus. So, if they need to bring a sizable force it will need atleast a month. Moreover, Most of the Nato forces in places like Poland and Germany have poor readyness. They will need a couple of weeks to prepare and train first before they are ready for a fight.
Then comes bringing those forces by rail and Train. You can't move heavy Abrams tank armies by Plane. That will take another 2 months.
Nato is not ready for even a conventional fight with Russia right now.
But all of this discussion is pointless because Russia will not allow Nato to intervene without using Tactical nukes. So, there goes your Nato Army.