They are already here.  
The enemy is already here.  
They are the enemy within.
If it’s terrorists that you are fearing.  Well, most likely they already reside within our borders.

But maybe it’s not terrorists that will bring down America.  Maybe it’s us - or at least some of us.

They are Americans (maybe in name only or maybe they just live here) but they do not have the will or wisdom to do what it takes to keep this country free.  Or maybe they just do not want this country to remain free.  At least free for the most of us.  Maybe they are only looking out for their own wealth & power.
See Charles Krauthammer’s article.

This section deals with the issue of who is our enemy.  We will know them by their fruits.  And some we can discern by their words alone.  Some will hard to believe.  Some will difficult to recognize.   Maybe.