Future U.S. political leaders – those for whom the Cold War was not the formative experience that it was for me – may not consider the return on America’s investment in NATO worth the cost.

Often, loyalty means telling people things they don't want to hear. It's not being sycophantic, it's not telling them how wonderful they are every day. It's being willing to tell them the days they're not wonderful.

Some people have said, in so many words, that I'm kind of wooly-headed in believing that the Iranians would see not having nuclear weapons as more in their security interest than not.

If there's ever an example that military power alone cannot be successful in Afghanistan, I think it was the Soviet experience.

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

I think that Vladimir Putin is certainly getting a lot of publicity for what the Russians are doing. And I'm not sure that's unwelcome to him. I think this is a guy who saw the U.S. basically come out against him in his reelection campaign in 2012. He saw the U.S. being behind all of the color revolutions in Eastern Europe and in Georgia and Ukraine and so on. So his view is the West has been interfering in his politics for years.

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