Mass killing morphs from criminality to terrorism when it is politically motivated and intended to produce political effects. With political violence, political grievances underpinning them must also be examined and countered or addressed: By our policies and actions, are we creating more terrorists than we are killing and capturing? The related question is: What would most profit the jihadis who want to recruit and motivate a growing army of angry Muslim youth to fight Westerners? The two biggest spurs are alienated and disaffected Muslims in Western countries and the sprawling presence of Western armies in Muslim countries.

Our intelligence must succeed every single time to thwart plots; the terrorists have to succeed only once to cause deadly mayhem. Within each plot, however, the terrorists have to succeed at every stage from start to finish. Intelligence has to succeed at any one stage of the plot to thwart it. Paris involved many agents in meticulous planning of simultaneous attacks on multiple sites. Their success represents a major intelligence failure. More extensive powers of surveillance may become necessary, but all increased police powers must be subject to independent oversight, periodic reviews and sunset clauses to preclude the rise of a police state as the new normal.

"With or without religion, you would have good people doing good things and evil people doing evil things. But for good people to do evil things, that takes religion," says Steven Weinberg. Most Muslims just want to live peaceably and quietly. Only a tiny minority are jihadis. Yet most terrorist incidents in the world today are perpetrated in the name of Islam.