When a rapidly rising power rivals an established ruling power, trouble ensues. In 11 of 15 cases in which this has occurred in the past 500 years, the result was war. The great Greek historian Thucydides identified these structural stresses as the primary cause of the war between Athens and Sparta in ancient Greece. In his oft-quoted insight, "It was the rise of Athens and the fear that this inspired in Sparta that made war inevitable."

One of the three big factors that count against war in the relationship between China and the U.S. are, first, nuclear weapons and even a condition of mutual assured destruction. Secondly, one's got two economies that have become so deeply interlaced that a war between the U.S. and China would leave Wal-Marts empty and Chinese factories producing for nothing. Thirdly, climate - if between the two of us, we keep doing what we're doing, we can create a climate in which our grandchildren won't be able to live.

When we succeeded in winning the Cold War, escaping a nuclear Armageddon that could have killed us all, the U.S. inevitably had a serious problem about an encore: what now for our place in the world?

There are two great tragedies in life: one is to fail to achieve one's grandest ambitions, and the other one is to succeed.

I think the election of Trump was an expression of what happens in ruling powers as they become alarmed.

Terrorism is really the only existential threat to America as we know it - as a free country that plays a leading role in the world.

Given that the Al Qaeda core has been significantly destroyed or disabled, and that the leadership seems to be essentially on the run or in hiding, it would lead me to think that probably it is one of the less centralized groups that might be the greatest danger.