Close runner-up to “world government” in the preposterosity stakes is the “proposition nation.” There never has been any such thing, nor could there be.
If a nation is to hold together, the great majority of its people need to be bonded by ethnic kinship—shared history, a shared outlook. One’s outlook arises from one’s brain, which is a product of evolution—including, we now know, historically recent evolution.
Because ethnies overlap on most traits, a particular Turk may become a good German—see the first item above. However, since we can’t tell in advance whether he will or not, a wise nation severely restricts settlement from foreign ethnies and vigorously deports those who prove incompatible.
A big ethnic minority with a different outlook spells national discord.