It was, as Putin said, an operation limited to ejecting Ukrainian and neo-Nazi forces out of Donbass, historically, like Crimea, a part of Russia herself and populated with ethnic Russians.  These historically Russian territories were transferred for political and administrative reasons from Russia to Ukraine by Soviet officials.  As Ukraine itself was historically part of Russia for centuries, and as both were part of the USSR along with Georgia and Central Asia, the transfer was understood as adjustment of state boundaries as in the US within the same country.  Ukraine did not exist as an independent country until about 30 years ago when the USSR was broken up.  So the claim that Donbass and Crimea are part of Ukraine has been true for about 30 years, whereas they have been a part of Russia along with Ukraine for centuries.

Putin had no choice.  The Minsk Agreement that he had devised, to which Ukraine agreed and Germany and France agreed to enforce, kept Donbass in Ukraine and gave the people semi-autonomy in order to protect them from abuse by Ukraine.  Both the German and French leaders who signed the agreement recently admitted that they used the Minsk Agreement to deceive Putin while the US and NATO built and equipped a large Ukrainian army capable of conquering the two self-declared independent republics in Donbass.  When Putin saw the large Ukrainian armed force ready to invade Donbass, part of which was already occupied by Ukrainian armed forces and neo-Nazi militias, he realized that he had been deceived by the West and had no other option but force to protect the Donbass Russians.