That's in the Commerce Clause. The power to regulate commerce with foreign nations covers anything coming across the border. There is ample historical reference to establish that as being within the original intent of the Commerce power.
Now, do you have anything to back up this "Defense clause" theory, or are you just making shit up?
You think people ship chemical weapons into this nation are engaging in commerce?
Perhaps you don't remember this, but a few years ago, Brian Williams (currently of ABC News) once shipped Uranium into the country to demonstrate how easy it was for terrorists to do it.
Do you think he was engaging in "Commerce"?
Do you think people sending Anthrax through the mail are engaging in Commerce?
I dare say "commerce" does not extend to the illegal shipment of dangerous/deadly substances across our borders. Such things are attacks on our people and our sovereignty.