There is no international problem that can be addressed or solved without the engagement and leadership of the United States and everybody in the world knows that, its just fact of life. So sometimes I think we could conduct ourselves with a little more humility.

I think it's kind of a stretch. You know, it's one thing if somebody working for the U.S. government has sold secrets to the other side, it's another if they have something in their personal life that they're hiding for which they could be blackmailed. Having evidence that they didn't tell the truth to somebody in the same building where they work, maybe it's just the old intel guy, is - it's a problem.

I will always be an advocate in terms of wars of necessity. I am just much more cautious on wars of choice.

It's clear the relationship between China and North Korea has hardly ever been worse. Kim Jong-un has never been to Beijing in his leadership. The Chinese press are saying some amazingly negative things about the north, and about Kim Jong-un. So - so they are weighing in, and they are bringing greater pressure. Whether it will be enough I think remains to be seen.

I mean, when you get down to very low numbers of nuclear weapons, and you contemplate going to zero, how do you deal with the reality of that technology being available to almost any country that seeks to pursue it? And what conditions do you put in place?

I had no difficulty as Secretary of Defense moving from the Bush administration to the Obama administration.

Well, what I've said is that the war in Iraq will always be clouded by how it began, which was a wrong premise, that there were in fact no weapons of nuclear - weapons of mass destruction.