Remember 9/11? Of course you do. Pearl Harbor? Sure.
But there are other types of attacks that can be used to bring a nation to its knees.
You can damage it economical
You can flood the country with people that cannot be identified
You can allow sick people to fly from one nation to another and spread disease.
For those of you that don't remember, we no longer regularly have outbreaks of diseases like measles and cholera and diptheria here in the United States because we require our kids to be inoculated