It says that in the Wong Kim Ark, 14th Amendment case. And that's the meaning of the "subject to the jurisdiction thereof" phrase. As the comments made at the time of the adoption of the amendment make clear, it was not meant to make a citizen of the child of every person who happened to wander into the U.S, and give birth. That phrase explicitly excludes those who are here as employees of a foreign power, such as diplomats. It also excludes tourists, whose loyalty is to their own country, not the U.S.