By the 20th century, war ceased to be an encounter between two armies. It became an encounter between two societies, because a factory worker producing a gun or a bomb is as deadly as a pilot.

If you go through the list of things that are not possible you're left with a very finite amount of possibilities. The fancy name for this is constraint theory. It's a nonquantitative model, but it's a field of mathematics.

When you have the countries like Germany, China, and Russia decline, and be replaced by others, that's when systemic wars start. That's when it gets dangerous, because they haven't yet reached a balance. So Germany united in 1871 and all hell broke loose. Japan rose in the early 20th century, and then you had chaos. So we're looking at a systemic shift. Be ready for war.

There has never been a century that has not had a systemic war - a systemic war, meaning when the entire system convulses. From the Seven Years' War in Europe to the Napoleonic Wars of the 19th century to the World Wars, every century has one.

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.

The British bombed German cities [during World War II] to keep the workers awake at night. So instead of dropping one bomb, we sent a thousand planes and, yes, we took out the factory sometimes, but we also took out the city. It reached the point where we wanted more efficient ways to destroy a city. The result was nuclear weapons.

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.