The fact that there are now many entities that may have some loose affiliation with a former core Al Qaeda - or who have decided to fashion themselves as an affiliate or follower in the Al Qaeda jihadist tradition - as well as groups that are just inspired by the concept that they could also be the perpetrators of mass killing, means that there is a spectrum of threats.

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

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."

For interests that are vital to the U.S. - that is, essential for our survival and wellbeing - the U.S. should be prepared to use military force - including unilaterally, if necessary.

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.

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?

If the Britain and France had done what they were obliged to do under the treaty and sent troops to enforce the treaty when Hitler remilitarized the Rhineland, the German general staff would have turned Hitler out. And one would not have had a cause for war, and you wouldn't have had World War II.