Politicians are enormously smart and rational. They don't have the same interests as businessmen ... But a man rises to the top of the United States. He's clawed his way out of 330 million people. OK. He didn't do that because he was dumb, or lucky, or something like that. He understands power. And he understands how to take it. And he understands how to keep it.

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.

Italy is the fourth-largest economy in Europe and the eighth-largest economy in the world, and its banking system is collapsing. And Germany is desperate. It must maintain its standard of living. It can only do that with exports and Deutsche Bank is very exposed to Italian debt. But so is the rest of Europe.

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

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.

In a way, we can have a much easier discussion about the future of technology than we can about why a young man kills another man in a war.

When I went to war, I did not go making geopolitical calculations. I went to war with a lust.