The identity issue is way more complicated than what ethnic group a person belongs to, which country's passport a person has or what language a person speaks.
In my trip in Europe, someone from the States argued that HongKongers are Chinese as we speak Chinese.
If that's true then majority of the States' population would be British because they speak English.
Maybe you live in a place where identity crisis is not an issue so you don't understand.
Is Obama a American or African? You can't identify him as an African simply because his skin is black. Many people in the world don't possess a passport. So where do they belong? You can see that identity, for some people, is not as straight forward as you might think.

The government of a country can change, boundary certainly always changes over time.
Australia was once owned by the indigenous Australian. The Roman Empire and Mongol Empire no longer exist. And imagine that you are living in a land occupied by the Nazis in WW2, are you automatically an Nazi once they have marched into your city?