Marco Rubio in ABC This Week interviews during 2015

On Foreign Policy: Too risky to take in Syrian refugees

RUBIO: The problem is we can't background check them. You can't pick up the phone and call Syria. And that's one of the reasons why I said we won't be able to take more refugees. It's not that we don't want to. The bottom line is that this is not just a threat coming from abroad. What we need to open up to and realize is that we have a threat here at home, homegrown violent extremists, individuals who perhaps have not even traveled abroad, who have been radicalized online. This has become a multi-faceted threat. In the case of what's happening in Europe, this is a swarm of refugees. And as I've said repeatedly over the last few months, you can have 1,000 people come in and 999 of them are just poor people fleeing oppression and violence but one of them is an ISIS fighter.

On War & Peace: Civilian causalities unfortunate but inevitable

RUBIO: I don't think any nation on Earth takes more pains in avoiding civilian casualties than the United States. The reality, unfortunately, is that many of these terrorist groups deliberately operate from the center of civilian areas, because they want there to be civilian casualties for propaganda use. We've seen that as well used by the enemies of Israel on repeated occasions. Obviously, we're going to take great pains to avoid civilian casualties, but at the end of the day, no one has killed more civilians and more innocents here than ISIS has. And although we'll take extraordinary steps to avoid civilian casualties, there is, of course, no guarantee, especially, given the fact, that you're operating against these individuals, who have no regard for human life.

On War & Peace: Declare war on ISIS, fight via Sunni tribes and Kurds

RUBIO: This is clearly an act of war on one of our NATO allies and we should invoke Article 5 of the NATO agreement and bring everyone together to put together a coalition to confront this challenge.

Q: The question is how--Senator Lindsey Graham, says put 10,000 troops on the ground.

RUBIO: I think it's premature to say the exact numbers. I think that we need to begin to work more closely with the Sunni tribes in Iraq who do not want to work under the thumb of the central government in Iraq as well as the Kurds. The only way to ultimately defeat ISIS is for them to be defeated ideologically and militarily, by Sunnis themselves. But we are going to have to increase special operations attacks, targeting ISIS leadership and revealing that they are not invincible.