So about Republic City. We know it was built on Earth Kingdom land. I think I recall the Earth Queen giving a shit about it. I don't recall ANY OTHER Earth Kingdom citizen giving a shit about it though. Like, using real life as an example, the Treaty of Versailles was a sticking point for the German population at large. Everybody felt bitter about it which of course made it easier for people like Hitler to gain power. How did Kuvira convince her troops or the Earth Kingdom at large that it was a good idea to take over Republic City? They did just 'cause reasons?
The anti republic city thing is something mostly echoed by the earth queen but the perception of losing territory existed in the comics where the various former fire nation colonies that seem to form much of republic city come under modest siege as the earth kingdom and fire nation almost go to war.