This is Revanchism - Putin wants to reconstitute the empire of Catherine the Great. The area he's taken from Ukraine is "Novorossiya".
Wiki:
The name received renewed emphasis when Russian President Vladimir Putin stated in an interview on 17 April 2014 that the territories of Kharkiv, Luhansk, Donetsk, Kherson, Mykolaiv and Odesa were part of what was called Novorossiya.[14][15][nb 2] In May 2014, the self-proclaimed Donetsk People's Republic and Luhansk People's Republic proclaimed the confederation of Novorossiya and its desire to extend its control towards all of southeastern Ukraine.[18][19][20] The confederation had little practical unity and within a year the project was abandoned: on 1 January 2015 the founding leadership announced the project had been put on hold, and on 20 May the constituent members announced the freezing of the political project.[21][22][23]
Anna Nemtsova forecast this disintegration in August 2014, and she predicted the 2022 Russian invasion of Ukraine then too.[24] Oksana Yanyshevskaya, a Ukrainian government official, in a July 2014 interview with her said that Novorossiya "is some sort of artificial idea that lives only in the minds of people in the Kremlin."[24]
Gerard Toal opines that "In breaking apart a sovereign territorial state, it is helpful, if not always necessary, to have an alternative geopolitical imaginary at the ready and for this ersatz replacement to have some degree of local credibility and support." The Novorossiya idea is just this portmanteau.[27][28]
The idea of Novorossiya goes hand-in-hand with the erasure of Ukrainian statehood,[29] or as Vladislav Surkov said in his defenestration interview in February 2020, "There is Ukrainian-ness. That is, a specific disorder of the mind. An astonishing enthusiasm for ethnography, driven to the extreme." Surkov claims that Ukraine is "a muddle instead of a state. [
] But there is no nation. There is only a brochure, The Self-Styled Ukraine, but there is no Ukraine."[30]