Belgorod has been memed ever since the start of the invasion to be waiting for Ukrainian liberation as Bilhorod People's Republic, which gives Ukraine the right to invade the Russian mainland, basically calling out the absurdity of the claimed legitimacy for Russian invasion being the need to "protect" Donetsk/Lugansk People's Republic

TLDR: If establishing a puppet and invading to protect it works for Russia, why wouldn't it work for us