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

My one concern is that Hu Jintao may overestimate how much power China has in Pyongyang. They have - they do have influence, and they do have companies, and they do have economic relationships that could make life much more difficult in the north. Their balancing act is, "How much worse can we make it in the north without creating that which scares us more than anything, which is a collapse in the north." And then what happens to all those nuclear weapons. So they're going to work very hard to avoid that.

There's a lot of books out there about how you lead change in business, but I've certainly not seen any... on how you do that in public institutions.

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.

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?

Well, I've ruffled a few feathers at all the institutions I've led. But I think that's part of leadership.

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.