Comment: The determination is made at birth.

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In reply to comment: so if a British man and (see in situ)

The determination is made at birth.

If the father holds British citizenship and the mother holds American citizenship, at the time of birth, the child (even though born on US soil) holds dual citizenship.

If the father became a naturalized American citizen before the birth of the child, the child is said to be natural born, born on US soil to US citizen parents.

Arguably, US military bases are US soil.