How about we just save a lot of time and you tell me where is the authority in the constitution for stopping people from shipping chemical weapons into the country?
Or do you think the government doesn't have the authority to do that?
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 There is ample historical reference to establish that as being within the original intent of the Commerce power.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?