COunter INsurgency. I believe Russia thought that Ukrainian government was weak so all they had to do was do what US did in Afghanistan or what they did in Syria: shock and awe with air power, go in with special forces, shoot up the commanders and organized resistance collapses. This works against poorly motivated insurgents but doesn't work against even inferior organized troops.
Ukraine is not that weak and the government isn't that unpopular though which is why their first assaults failed.
Remember that even when Nazi Germany steamrolled Poland in what is regarded as basically destroying an unarmed opponent as a war crime, they still lost 17k soldiers, 270 planes and 1000+ vehicles even though they had far more planes, vehicles and soldiers. Poland was far smaller and poorer than Germany at the time. Russian expeditionary military is actually smaller than Ukrainian.