Which one? There are a number of rulings, some going back over 100 years, that agreed that children born here are, with few exceptions, U.S. citizens regardless of the citizenship of their parents.
Simple: United States v. Wong Kim Ark. Because this is the one that decided that as long as you are on American soil you are an American. Even though it should be pointed out that in this case, his parents were LEGAL non-citizens!
As was stated in the dissenter’s opinion for this case. In the view of this decision, those children born to non-citizens in America on vacation could become President, but those children born to American-citizens who are in Europe for vacation would NOT be allowed! Of course, starting with our present President, none of this seems to matter anymore!
All I know is that there is more to being an American than where you place your feet when the baby hits the ground!