The
Indian Citizenship Act of 1924 (43
Stat. 253, enacted June 2, 1924) was an act of the
United States Congress that declared
Native Americans born within the United States are
US citizens. Although the
Fourteenth Amendment to the U.S. Constitution provides that any person born in the United States is a citizen, there is an exception for persons not "subject to the jurisdiction" of the federal government. This language was generally taken to mean members of various tribes that were treated as separate sovereignties: they were citizens of their tribal nations.
en.wikipedia.org
Being born off the reservations did not change their status as members of their Indian tribes. Mexican babies born to two legal Mexican citizens are considered citizens of Mexico regardless of where they are born. Mexico is a sovereign country.