Immigration is the most difficult issue I've ever dealt with, and I've dealt with some tough issues: drones, gays in the military, WikiLeaks, Guantanamo. But immigration is hardest because there are so few people willing to talk and build consensus. Everybody's firmly made up their mind. It's a polarized issue.

Under well-settled legal principles, lethal force against a valid military objective, in an armed conflict, is consistent with the law of war and does not, by definition, constitute an 'assassination.'

In my view, targeted lethal force is at its least controversial when it is on its strongest, most traditional legal foundation. The essential mission of the U.S. military is to capture or kill an enemy. Armies have been doing this for thousands of years. As part of a congressionally authorized armed conflict, the foundation is even stronger.

ISIL, AQ, now have the ability to literally reach into our homeland through social media, through the Internet, to recruit and inspire. It makes for a more complicated homeland security environment. And so it requires a whole of government approach, not just military and law enforcement, homeland security, aviation security, and the like.