When you're young and going to war, it's a genuinely exciting moment. You are going to risk yourself. On the battlefield, you are suddenly free. You realize: I'm here, I'm in it. Exaltation. Suddenly you're hit by another extraordinary feeling: my God, I can be killed. And: will I embarrass myself? It's like you're in a kaleidoscope and all of these extraordinary feelings are zipping by.

War to me is very much like sex. You can develop a theory that says sex is primarily for the exchange of genetic material, or that it's a celebration of life, or you can make up 50,000 theories about why human beings have sex, all of which are in some sense true, all of which by themselves miss the point. Because the answer is extraordinarily complex. It may be for fun, it may be for reproduction, for financial reasons, any number of things.

Constraint theory argues a number of things. First, that the impossible has to be identified. Second, that the actor is then constrained by circumstances to act a certain way. For example, should we invade ISIS? Can we invade ISIS? What would it take to invade ISIS? Once you ask that question you discover the price of that option and then you take a look at American politics and see that the country is probably not prepared to invest the 2 to 3 million people that it would take to defeat ISIS and the insurgency afterwards. All right, so that's not going to happen.

Because the US has control of the sea. Because the US has built up its wealth. Because the US is the only country in the world really not to have a war fought on its territory since the time of the Civil War ... Therefore we can afford mistakes that would kill other countries. And therefore we can take risks that they can't ... the core answer to why the United States is like this is we didn't fight World War I and World War II and the Cold War here.

Well what are our geopolitical objectives? First, that North America be peaceful, prosperous, dominated by the United States. Second, that no nation be able to approach the United States militarily ... Those are the goals. It's very simple. We achieve that by making certain that all conflict takes place in the Eastern Hemisphere so we don't have conflict here.

The importance of the question and the availability of an answer are two different things. I'm not willing to state that because the question is fundamental, therefore I possess the answer. And I'm certainly not willing to say that since I don't possess the answer, I'll pretend that I do.

I think there are worse things than war. For example, injustice.