As we know, the US Constitution provides that only a US citizen born on US soil can be elected US president. How about a hypothetical scenario of a US citizen born on a US military base abroad?
Children born to US citizens are considered 'natural' US citizens regardless of where they are born. If US citizen has a child at an overseas military base, or in a foreign country, or even on Mars, there would be no constitutional obstacle to that child becoming president.
The question of 'US soil' only arises when a woman who is not a US citizen (or married to a US citizen) has a child. If that child is born on US soil it is considered a 'natural' citizen, if not, not. Military bases are not considered US soil in the same way that (say) an embassy or consulate is, so children born on a base to non-citizens are not automatically granted citizenship. They would have to be naturalized, which would bar them from being president.
Military bases are not US soil; neither are US embassies or consulates. Here's what the US State Department's Foreign Affairs Manual has to say about it (8 FAM 301.1-3 NOT INCLUDED IN THE MEANING OF "IN THE UNITED STATES"):
c. Birth on U.S. military base outside of the United States or birth on U.S. embassy or consulate premises abroad:
(1) Despite widespread popular belief, U.S. military installations abroad and U.S. diplomatic or consular facilities abroad are not part of the United States within the meaning of the 14th Amendment. A child born on the premises of such a facility is not born in the United States and does not acquire U.S. citizenship by reason of birth;
(2) The status of diplomatic and consular premises arises from the rules of law relating to immunity from the prescriptive and enforcement jurisdiction of the receiving State; the premises are not part of the territory of the United States of America. (See Restatement (Third) of Foreign Relations Law, Vol. 1, Sec. 466, Comment a and c (1987). See also, Persinger v. Iran, 729 F.2d 835 (D.C. Cir. 1984).
Furthermore, the foreign-born child of a US citizen normally acquires US citizenship at birth and therefore never needs to be naturalized and is therefore a natural-born US citizen eligible to be president. There are those who argue otherwise, but they generally rely on The Law of Nations by Vattel, which is not a direct source of US law, and ignore the fact that British law explicitly included foreign-born children of British subjects as "natural-born subjects." They also ignore Vattel's assertion that foreign-born children follow the condition of the father unless the father has "fixed his domicile in a foreign country."
Well, Wikipedia says of the best known example (mentioned in a comment):
Had he been elected, [John McCain] would have become the first president physically born outside the United States. This raised a potential legal issue, since the United States Constitution requires the president to be a natural-born citizen of the United States. A bipartisan legal review, and a unanimous but non-binding Senate resolution, both concluded that he was a natural-born citizen. However, other legal scholars came to the opposite conclusion that although he was a citizen, at the time of his birth he was not a natural born citizen; that is because the 1937 law that made him a citizen was passed one year after his birth.
Which cites a NYT article for those inclined to pursue this. So, the political will was there to declare McCain eligible, but who knows what might happen in a different case, as circumstances would probably not be 100% identical (given that that law was specific to Panama--see 8 FAM 302.4).
I.e. it's possible for someone to be made retroactively a citizen by law. Whether that would also satisfy the judicial branch for the purpose of making someone "natural born Citizen" is not entirely clear (because that issue is rather untested in courts)... but Wikipedia also says:
The U.S. Constitution uses but does not define the phrase "natural born Citizen" and various opinions have been offered over time regarding its exact meaning. The consensus of early 21st-century constitutional and legal scholars, together with relevant case law, is that natural-born citizens include, subject to exceptions, those born in the United States. As to those born elsewhere who meet the legal requirements for birthright citizenship, the consensus emerging as of 2016 was that they also are natural-born citizens.
For the latter it references a legal challenge to Ted Cruz, dismissed by the Supreme Court of Pennsylvania in 2016:
Texas Sen. Ted Cruz has won a case in Pennsylvania’s highest court that had challenged his eligibility to appear on the state’s GOP primary ballot and serve as president.
The state Supreme Court order Thursday upheld a lower-court judge’s decision to dismiss the case.
A Pittsburgh resident and registered Republican voter, Carmon Elliott, had argued that Cruz isn’t eligible to run for president or to appear on Pennsylvania’s April 26 primary ballot because he was born in Canada.
Commonwealth Court Judge Dan Pellegrini ruled March 11 that common law precedent and statutory history maintain that an eligible candidate includes any person born to an American citizen, regardless of where.
Elliott had acknowledged that Cruz’s mother was born in the United States and has been a U.S. citizen her whole life.
But, of course, another judge in some other US state could rule differently, with SCOTUS as the ultimate arbiter.
As we know, the US Constitution provides that only a US citizen born on US soil can be elected US president.
False, there is no provision of the US Constitution that clearly states such a requirement.
What it actually provides for is that
No Person except a natural born Citizen, […] shall be eligible to the Office of President;
The Constitution did not come with a glossary, and the founders did not define for us exactly what “natural born Citizen” meant to them. One interpretation says that it means “a US citizen born on US soil,” but that is not the only interpretation of the phrase. Children born abroad can be born with US citizenship in some circumstances,¹ meaning they were born with that citizenship and so might—or might not—count as a “natural born Citizen […] of the United States” as required by the Constitution.
In the absence of clarity in the Constitution itself, this issue would have to be settled by the courts. To date, it has not been fully settled once and for all, but we have some information to go on: at least two people who have run for president have had questionable claim to being “natural born Citizen[s],” John McCain (born on a US military base to two US-citizen parents) and Ted Cruz (born in Canada to a US-citizen mother).
John McCain’s case is arguably the more relevant of the two, since he was actually born on an overseas military base, just as in the question. However, there are a lot of particular details in his case that are not necessarily relevant today—the fact that he was born specifically in a US military base in the Panama Canal Zone is significant, as is the fact that his citizenship was derived from a 1937 law—passed a year after he was born. Moreover, no court ruled on the matter—Congress passed a non-binding resolution acknowledging his qualification to be president, but the legal challenges that were made got dismissed for procedural reasons. Had McCain been elected, it is likely that the matter would finally be put before the Supreme Court.
Ted Cruz’s situation actually saw at least one court rule on the matter, though since it was not the Supreme Court, it could still be challenged. In his case, Cruz was unaware of his dual Canadian citizenship right until 2014; he renounced that citizenship and none of the numerous challenges to his candidacy prevailed. In each case, the ruling by the court or board or whatever was that the fact he had US citizenship from birth—even though he was born in Canada and also had Canadian citizenship from birth—meant he met the Constitutional requirements to be president.
- See Acquisition of U.S. Citizenship at Birth by a Child Born Abroad from the State Department.