Failing this, he needed to reincorporate Donbass in 2014 prior to its partial occupation by neo-Nazi militias and Ukrainian military.  He failed to act in a timely fashion because he was deceived by the West with the Minsk Agreement, which was a delaying tactic  to keep Russia at bay until the US had built and equipped a Ukrainian Army.

I understand why the Kremlin limited its intervention last February to clearing the Independent Donbass Republics of Ukrainian forces. Putin did not invade Ukraine, because he did not want widespread death and destruction in Ukraine, long a part of Russia with extensive intermarriage and mixing of populations.  Where the Kremlin failed was in not realizing that Washington would not permit the conflict to be limited to the clearing of Donbass.

Having delayed necessary action for eight years, the Kremlin needed a lightening, overwhelming  attack that knocked the entirety of Ukraine out of action immediately before Washington and NATO could get involved.  Instead the limited military operation was confined to the territory of the breakaway republics, proceeded slowly to spare civilian lives and infrastructure, and involved little of the Russian Army.  The fighting was undertaken by the Donbass militias and a private military organization, the Wagner Group, that functions as an auxiliary to the Russian Army.  The Kremlin did not have ready any reserves to maintain the ever-lengthening lines of the slow Russian advance that gave Washington time to widen the conflict.  It was this that made possible the limited Ukrainian "counterattacks" that had no meaningful effect.  The Russian lines in two sectors pulled back in advance and made penetration impossible.