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.

What is the great fear of the United States? That an Eastern power will build a navy to challenge us. How do you keep them from doing that? Keep them at each other's throats so they don't have any money to do this. This is why we fought the First World War, the Second World War, and the Cold War.

It has always struck me as the world's great fortune that the two great superpowers were the United States and the Soviet Union, who managed the Cold War with meticulous care in retrospect. Imagine the European diplomats of 1914 or 1938 armed with nuclear weapons. It is easy to believe they would not have been as cautious.

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.

Wars are times of intense technological transformation, because societies invest - sometimes with extensive borrowing - when and where matters of life and death are at stake.

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.

Constraint theory defines for you what outcomes are possible and what outcomes are impossible. It also eliminates wishful thinking.