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Trump’s mother was a British subject. Obama’s father was a Kenyan.
There you go.

Legally, there are different bases for citizenship, Jus sanguinis (by the blood from the parents), jus soli, (by the place of birth), us matrimonii, (by marriage). American women took the citizenship of their husbands, and lost the citizenship of their birth, by the Expatriation Act of 1907 until it was repealed in 1922. People also can have more than one citizenship as more than one state can claim a person as its citizen. There are treaties dealing with the issue, especially as it pertains to compulsory military service. I have heard stories of ethnic Greeks not visiting Greece for fear of being conscripted. The idea of dual citizenship blows people's minds because they think that a person can only have one citizenship. Nothing prevents Kenya from calling Obama its citizen, but that doesn't change his mother having been an American citizen regardless of where he was actually born.