The a US Circuit Court in the case of US v. Rhodes defined natural born citizenship as birth within the allegiance of the United States [cited approvingly by the Supreme Court in US v. Wong Kim Ark]. Some have said that because President Obama, through his father, was born also a Citizen of the UK and Colonies, that he has a dual allegiance to Britain, and that this defect renders him not a natural born citizen of the United States.
It has always seemed to me profoundly unfair that some second country could impose an allegiance upon someone against their will, or that somehow a second country could dictate who could and who could not be President of the United States. Apparently, the court agreed with me in this decision from the case of Calais v. Marshfield (1844):
“Although the government of one country may grant to persons owing allegiance to that of another, the rights and privileges of citizenship, it is not intended to intimate that the government making such grant would thereby, and without their consent or change of domicil, become entitled to their allegiance in respect to any of their political duties or relations.” Calais v Marshfield 30 Maine Rep 520. [As cited in Fields International Code.]