Neither could the US.
Another factor here is that Ukraine isn't Afghanistan or Vietnam where natural barriers offer chance for insurgency to hide out, regroup.
Russia here can absolutely level cities and buildings and it is impossible to construct underground tunnels in flat lands known for its ability to soak up water (the tunnels would not be structurally safe), and it is far too late now in this stage of the war.
An incompetent large army vs a competent large army makes little difference when the fighting environment is against the defenders (no mountains or jungles to hide out in).
The best chance at insurgency would be to start digging but again the soil in Ukraine makes this a challenge, my prediction is that we could see something similar to what Hamas is doing in Palestine in the form of human shields, swaying public media through casualties, urban warfare at best but unlike IDF which does to a certain degree exercise in minimization of collateral damage (ex. calling occupants in the apartment building to evacuate before bombing Hamas launching rockets) where as Putin/RF has zero qualms about collateral damage.
The Western sanctions have only esclated tensions, have emboldened Putin even more, as they've given him nothing to lose. When you corner a wild animal like Putin you will get bitten.
There's just no way out for him except to escalate from what I can see since Ukraine has rejected giving up DNR/LNR/Crimea. Neither side is willing to budge so this will simply continue to lead to human suffering on both sides.