Birthright citizenship.
Comes from the Citizenship Clause of the 14th Amendment of the Constitution: “All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the state wherein they reside.”
It was enacted, and has since been interpreted, to protect the citizenship rights of those of African, Chinese, and other ethnic minority descent because there have always been those who have considered us illegitimate Americans. It was originally passed by Congress after the Civil War to cancel out the US Supreme Court Dred Scott decision of 1858 which said that African Americans were not entitled to US citizenship. Even after the passage of the 14th Amendment, in 1898, some attempted to deny birthright citizenship to Chinese Americans in the US Supreme Court case of Wong Kim Ark. Anti-Chinese activists claimed that the Chinese Exclusion Act (which was in effect officially from 1882-1943, and practically until 1965) nullified birthright citizenship for Chinese Americans. Thankfully, the US Supreme Court disagreed and applied birthright citizenship to Chinese Americans. All Native Americans, however, were not granted citizenship until 1924. On a related note, from 1790-1952, non-whites were barred from becoming naturalized US citizens.
And so, birthright citizenship, and citizenship in general, have been a hard fought civil rights struggle in the United States for well over 150 years.
The settled law for over a century has been: if you are born in the US, then you are a citizen.
Because the concept of birthright citizenship derives from the Constitution, the current president cannot just wipe it away unilaterally through an executive order as he has tried to do. That would be like a president saying unilaterally that he/she is canceling out the constitutional rights to freedom of speech, freedom of religion, or the right to vote.
The Constitution can only be changed by a two-thirds vote in both the House and Senate, together with ratification by three-quarters of the states.
Absent such congressional action, the only other path for the executive order to stand is for the US Supreme Court to issue a bizarre ruling which radically departed from its own well established precedent for over a century. I suppose stranger things have happened, but it is still quite unlikely.
Welcome back to 1858.