Legitimacy is in my opinion just a concept that has been invented by a ruling power to disenfranchise a secessionist state. It stems from the feudalistic times when the legitimacy of a heir was crucial. Rivaling nobles both claiming to be the heir. That’s where legitimacy originally came from.

This concept has persevered through time, somehow, even though nobles are a thing of the past. The only metric that can somewhat measure legitimacy is whether the citizens inside are in favor of the state as such, as they usually form the state. However, even this does not decide legitimacy. Even a dictator-state with a population that hates him can have legitimacy. Purely because that state exercises real power over a region and its legitimacy isn’t contested by other states.

Let me take North Korea as an example. It started out as a people’s revolution, at the time you would say that North Korea would be legitimate in that sense. South Korea has legitimacy as it was the state that controlled the region up until that revolution. If we lived in a perfect world, the revolution would have been completely succesful and the legimitacy would have fully transfered from ROK to DPRK. However, we dont. Both hold some form of legitimacy, ROK as the original ruling state, and DPRK as the people’s preference at time of the revolution. Currently, ROK, too, has some legitimacy coming from its population, since it transfered from a dictatorship to a democracy, increasing its popularity among the population.

So my view is that both can be legitimate in different ways. Then it comes down to who de facto rules the land. Other countries recognize whoever they prefer geopolitically. But picking a side is inherently biased and ignores and discards the legitimacy of the rival state. Only people led by propaganda will pick a side.

Other factors include genocide, historical claims, ethnic differences, religious differences etc etc