He wasn't.
When our founders formally adopted the U.S. Constitution, they granted U.S. citizenship to themselves and the citizens of each state within the colonies. Prior to that, there was no such thing as U.S. citizenship.
When the founders wrote Article II, Section 1, Clause 5, they acknowledged that there were then no natural-born citizens of the U.S., which is why they grandfathered themselves into being eligible for the presidency.
No Person except a natural born Citizen, or a Citizen of the United States, at the time of the Adoption of this Constitution, shall be eligible to the Office of President; neither shall any Person be eligible to that Office who shall not have attained to the Age of thirty five Years, and been fourteen Years a Resident within the United States.
I meet those requirements.
Vote for me LOL
Seriously, I think it sucks that conservatives are looking for a reason to disqualify Cruz.
The guy has just gotten started and I like what I see.