He's co-opting the red army's defeat of Nazi Germany for his own popularity purposes. Which is impressive, considering he's also disavowing communism. It would hardly have been possible, if it weren't for fringe (but not fringe enough) movements in Eastern Europe playing along with it. Not because they're pro-Russian, far from it, but because their old nationalist groups often were aligned with the nazis, and they want to rehabilitate them. Putin and these groups totally agree that the conflict should be framed as being between Russia and these groups.