From the Minor decision:
"Before its adoption the Constitution of the United States did not in terms prescribe who should be citizens of the United States or of the several States, yet there were necessarily such citizens without such provision. There cannot be a nation without a people. The very idea of a political community, such as a nation is, implies an [p166] association of persons for the promotion of their general welfare. Each one of the persons associated becomes a member of the nation formed by the association. He owes it allegiance and is entitled to its protection. Allegiance and protection are, in this connection, reciprocal obligations. The one is a compensation for the other; allegiance for protection and protection for allegiance.
For convenience it has been found necessary to give a name to this membership. The object is to designate by a title the person and the relation he bears to the nation. For this purpose the words "subject," "inhabitant," and "citizen" have been used, and the choice between them is sometimes made to depend upon the form of the government. Citizen is now more commonly employed, however, and as it has been considered better suited to the description of one living under a republican government, it was adopted by nearly all of the States upon their separation from Great Britain, and was afterwards adopted in the Articles of Confederation and in the Constitution of the United States. When used in this sense it is understood as conveying the idea of membership of a nation, and nothing more."
Each one of the persons associated becomes a member of the nation formed by the association. He owes it allegiance and is entitled to its protection. Allegiance and protection are, in this connection, reciprocal obligations. The one is a compensation for the other; allegiance for protection and protection for allegiance.
In other words, if one is a dual citizen, like Mr. Obama is, he owes his allegiance to both the United States AND the United Kingdom. This would pose an even larger problem if he were royalty. He would be unable to recind his citizenship...ever. Fortunately, Mr. Obama is not royalty. However, others are, and they too were born in the United States, and would be considered eligible to become president, according to recent court opinions.