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

On the foreign policy side, there's the risk of being too spontaneous and too disruptive where you end up doing more harm than damage. And figuring out that balance is where having strong people around you matters.

Even when I was at CIA, I'd go to visit foreign leaders and I'd say, 'You know, I'm not a diplomat. I'm just an old CIA guy'... I said, 'If I wanted to be diplomatic, I'd have been a diplomat.'

I think when it comes to the issues, I'd advise him to stick to the script. But, I mean, Donald Trump is going to have some very tough conversations and he's going to be talking about some very tough and complicated issues in all of the places that he visits.I think anytime a president does things that are humanizing, I think it's - it's good.

One of my favorite little sayings is, 'To avoid criticism, say nothing, do nothing, be nothing.

My last visit to China as secretary, January of 2011, I told President Hu Jintao, just like this, "President of the United States wanted me to tell you that we now consider North Korea a direct threat to the United States." And it had no effect whatsoever.

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