How can Cruz be a natural born Canadian citizen (born in Canada) and a natural born US citizen? How can Cruz be a natural born Cuban citizen (father was Cuban citizen) and a natural born US citizen? Citizen at birth is not the same as natural born citizen.
Because (as noted by Bayard and approved by Chief Justice Marshall) "natural born citizen" only requires a person be a citizen at or by birth, and persons born US citizens overseas are eligible to be elected President.
This isn't any recent doctrine. Bayard wrote his Exposition of the Constitution in 1834.
There are people born on US soil, to citizen parents, who are BORN dual citizens with another country. Doesn't matter in the slightest. The laws of another country don't and can't specify whether a person is a natural born UNITED STATES citizen.
Don't let the birther propagandists tell you such dual citizenship disqualifies a person, either. It doesn't. That's not part of the specification.
In fact, 3 of our first 4 Presidents were dual citizens... WHILE SERVING AS PRESIDENT.
George Washington, Thomas Jefferson, and James Madison were all citizens of France while serving as President. Such citizenship was given to them by the French legislature. Not one of them renounced it.
The Vattel birther meme is simply a false one, propped up by nothing more than propaganda and false arguments. It's been a false meme from the beginning, and it's time it ended.
Especially at a site where people value our history and our Constitution.