I don't mean that the Kremlin will fail in its effort to drive all Ukrainian military units out of Donbass.  The mistake the Kremlin made was in thinking that the military intervention could be limited. 

The US and NATO are now involved in the conflict.  Ukraine is being supplied with heavy weapons and with training in their use.  Washington has now arranged to send missiles capable of hitting Russian territory.  Clearly, there is nothing limited about the conflict.

The dilemma for Russia is that by confining its military operations to the Donbass region in east Ukraine, Western weapons are accumulating in west Ukraine where it is possible to raise an army to replace the one the Russians are destroying in Donbass.  Whereas the Kremlin will succeed in liberating Donbass, it will fail in its goal to demilitarize Ukraine and turn Ukraine into a neutral state unless the intervention is expanded to all of Ukraine.  

Additionally, the conflict has also expanded by the request of neutral Finland and Sweden to join NATO. The addition of Finland to NATO would greatly expand the presence of NATO on Russia's borders, which the intervention in Ukraine intended to prevent.

The Kremlin warned that countries that intervene in the conflict would be treated as combatants, but has failed to take any action. The Russian threat is so disregarded that even militarily insignificant Denmark openly sends weapons to Ukraine.  The consequence will be more intervention by the US and NATO.