One of the few roles a "small government" should actually take on is defending from invaders.

You are misled/misleading. Get informed about the political theory of domestic deployment of troops for the purpose of policing in western democracues. Look how those who speak for the U.S. military personell (former generals, the editorial of that magazine they produce in the U.S. army prior to being pruned) if you need some motivation.

Are the invaders in the room with us right now?

Which invaders? Has the Iran War taken a 180 degree turn?