The situation in these areas is substantially more complicated than that. They're part of Ukraine's internationally recognized territory, yes, but they seceded

The situation in these areas is substantially more complicated than that. They're part of Ukraine's internationally recognized territory, yes, but they seceded in response to a 2014 overthrow (very arguably a coup) of the democratically elected president they supported. So there are good reasons that a Ukranian military takeover would be seen as aggression.