Same M.O. as Hitler : all people of Russian ethnicity AND the places they live must be under one flag...
Not necessarily all, but Putin wants Donbas and southern Ukraine to provide a land connection between Russia and Crimea and Moldova. When you look at the map it makes sense. The fact that it leaves Ukraine landlocked is seen as a bonus that in time may bring all of Ukraine back into the fold. But this is why he may declare victory without conquering all of Ukraine at this time.