My grandfather (mother's side) was 100% Spaniard but married a non-spaniard.

AMERICAN CITIZEN SERVICES
Dual Nationality

Dual nationality is the simultaneous possession of two citizenships. The Supreme Court of the United States has stated that dual nationality is "long recognized in the law" and that "a person may have and exercise rights of nationality in two countries and be subject to the responsibilities of both…”

Dual nationality results from the fact that there is no uniform rule of international law relating to the acquisition of nationality. Each country has its own laws on the subject, and its nationality is conferred upon individuals on the basis of its own independent domestic policy. Individuals may have dual nationality not by choice but by automatic operation of these different and sometimes conflicting laws. For example, a child born abroad to U.S. citizens may acquire at birth not only American citizenship but also the nationality of the country in which he was born. Similarly, a child born in the United States to foreigners may acquire at birth both U.S. citizenship and a foreign nationality.

The automatic acquisition or retention of a foreign nationality does not affect U.S. citizenship; however, the acquisition of a foreign nationality upon one's own application or the application of a duly authorized agent may cause loss of U.S. citizenship. United States law does not contain any provisions requiring U.S. citizens who are born with dual nationality to choose one nationality or the other when they become adults. While recognizing the existence of dual nationality and permitting Americans to have other nationalities, the U.S. Government does not endorse dual nationality as a matter of policy because of the problems it may cause. Claims of other countries upon dual-national U.S. citizens often place them in situations where their obligations to one country are in conflict with the laws of the other. In addition, their dual nationality may hamper efforts to provide diplomatic and consular protection to them when they are abroad. It generally is considered that while dual nationals are in the other country of which they are citizens that country has a predominant claim on them.

Like Americans who possess only U.S. citizenship, dual national U.S. citizens owe allegiance to the United States and are obliged to obey its laws and regulations. In cases where dual nationals encounter difficulty in a foreign country of which they are citizens, the ability of U.S. Foreign Service posts to provide assistance may be quite limited since many foreign countries may not recognize a dual national's claim to U.S. citizenship.

Except in certain circumstances, U.S. citizens must use U.S. passports when entering or leaving the United States. Dual nationals may be required by their other country of citizenship to enter and leave that country using its passport. Complying with this requirement does not endanger the dual national’s U.S. citizenship.

Generally, persons who do not wish to maintain dual nationality may renounce the citizenship they do not want. Information on renouncing a foreign nationality may be obtained from the foreign country's Embassies and Consulates or from the appropriate governmental agency in that country. Information on renouncing U.S. citizenship may be obtained from U.S. Embassies and Consulates and the Office of Citizens Consular Services, Department of State, Washington, DC 20520.

There is no reason for us, born citizen of the US, to go overseas and live an uncomfortable & deceiving life.