In 1864, Attorney General Edward Bates tackled the issue in connection with African-American members of the Union Army, finding that “free men of color” born on American soil were American. After the war, the Reconstructionist Congress passed a civil rights
law that extended citizenship to all people born in the U.S. who were
“not subject to any foreign power.”
The 14th Amendment extended birthright citizenship
The most sweeping declaration of birthright citizenship came in 1868: the
Fourteenth Amendment. It defined citizenship as applying to “All persons born or naturalized in the United States, and subject to the jurisdiction thereof.” Notably, due to the "jurisdiction" requirement, most Native Americans were excluded from this amendment.
Why the United States Has Birthright Citizenship