Putin assumed Ukraine was as weak as 8 years ago.
He assumed he could compel on the battlefield what had thus far eluded him at the diplomatic table - declared neutrality from Ukraine, recognition of Crimea as Russian, and either independence for Donetsk and Luhansk, or more suitably from the Russian perspective, membership in a federated Ukraine serving as a Russian bloc in Ukrainian politics.
They expected something akin to the US invasion of Panama. A few days of shooting and then the process of installing a new regime.
Instead they had split their forces in the face of well armed and determined enemy, squandering their numerical superiority, and putting extra burden on a clearly insufficient logistics train.
“Klotzen, nicht Kleckern!” as Heinz Guderian would say.