Babies born to foreign diplomats stationed in the U.S. do not become U.S. citizens. The words of the Fourteenth Amendment, "subject to the jurisdiction thereof" do not apply to those in the U.S. with diplomatic immunity.
Similarly, at least some U.S. military stationed in foreign countries are there under the provisions of treaties with those countries and are not completely "subject to the jurisdiction" of those countries. Also, such people are perhaps completely "subject to the jurisdiction" of the U.S. government.
Thus it is completely possible that a child born to a U.S. military couple located in a foreign country does not gain the citizenship of that country. To suggest that such a child becomes a "stateless person" because of the location of his two U.S. citizen parents is, I think, a conclusion without merit.
Of course John McCain was born a US citizen but is a he a natural born citizen? Which is necessary to be president and vice-president