If you only care about the current citizens'/government's opinion, you actually can ignore history leading up to that point. Geopolitics is complicated, however, and I think most people would bring more nuance to any moral judgment. What if a large portion of the previous population was killed/driven out, for instance? What if the citizens are subjected to "brainwashing"?

I do have to disagree on the notion that a government should have a historical or traditional basis to be legitimate. In my eyes it needs foremost the support of its "subjects". In reality, different factors make a stable government, but legitimacy is ultimately a value judgment.