Big Serge has a new piece up. Some major points on why Ukraine lost:
Inability to commit a mass of forces. Ukraine lost in large part because they failed to commit a large enough force in any single battle, instead resorting to a small trickle of forces that were never enough to overcome enemy defenses (why they never made it through even the first line of defense to the main battle line).
Russia readily targeted Ukrainian troops with its artillery and air force as they were assembling, and Ukraine's response was to spread out into smaller units no larger than a company and disperse along the line. Such forces were never able to make any significant gains.
Contrast that with Russia's recent pincer move to gain height over Avdiivka--it sent a large army to take the heights, exposing it to fire, and taking losses, but it was worth it to take the high ground which they now hold and can bombard both the main Ukrainian army and any attempts to resupply (or evacuate) it.
American strategic support was flawed (both in its plan to hurt Russia through sanctions, which only strengthened it, and through flawed tactical advice and training).
Ukraine attacked exactly where Russia expected them to. Deception is harder in the era of mass surveillance, satellites, and drones, but look at what Russia did with the Kiev "our tanks ran out of gas" fakeout at the beginning of the conflict.
Superweapons are overrated (Nazi Germany had these in almost every category but eventually lost the war when the totally of allied forces presented too great industrial and manpower for a single nation to resist).
Inadequate combat engineers: Russian minefields were a major deterrent and Ukraine had no good answer for them.
What's next?
Avdiivka will fall, but not before Zelensky sends in many more troops in a hopeless attempt to save it. Once Russia kills them, the city will fall, possibly in late winter.
Ukraine will continue to weaken its army by redeploying troops across sectors, taking snipers and medics and engineers and throwing them on the front lines as assault infantry, where they are less effective, and leaving very few of the specialty troops.
The west will be unable to supply Ukraine as well as it did this year. Europe and America are facing economic problems, and even if their leaders chose to ramp up military production at the expense of domestic, the costs of a privatized and zero-inventory capitalistic supply line system will bring huge costs (shells that the army was paying $800 for are now going for $2,300, due to both corporate greed, shortages of labor, and higher material costs). And of course the first priority is now Israhell.
In military affairs there are five essential points. If able to attack, you must attack. If not able to attack, you must defend. If not able to defend, you must flee. The remaining two points entail only surrender or death.
Ukraine is working its way down the list. The events of the summer demonstrated that it cannot successfully attack strongly held Russian positions. Events in Avdivvka and elsewhere now test whether they can defend their position in the Donbas against rising Russian force generation. If they fail this test, it will be time to flee, surrender, or die.