Columbia subject headings:
Harvard subject headings:
Free ports and zones--Mexico--History.
Free ports and zones--Rio Grande Valley (Colo.-Mexico and Tex.)--History.
Free ports and zones--Rio Grande Valley--History.
Mexico--Foreign economic relations--United States.
Mexico--Foreign relations--United States.
United States--Foreign economic relations--Mexico.
United States--Foreign relations--Mexico.
Princeton subject headings:
Mexico--Foreign relations--United States.
Mexico--History--1867-1910.
United States--Foreign economic relations--Mexico.
United States--Foreign relations--Mexico.