The Chinese, for their part, are thought by many to be equally in the dark about Washington’s program and about the United States more generally.

It is, again, impossible to say what exactly President Clinton was thinking in 1998, unable to anticipate the massacre of 2001. But it would not be unreasonable to assume that he was thinking a lot more about Monica Lewinsky and Newt Gingrich than he was about Osama bin Laden and Mullah Omar.

If the U.S. economy is deep in recession a year from today, what will President Trump be thinking about? Buying Greenland? It is not impossible to imagine Bill Clinton’s using military strikes to distract from his adultery with a young White House intern. What might Donald Trump do with GDP growth in the negative or gasoline at $6 a gallon? What might Xi Jinping be tempted to do if the scene on the street in Beijing starts to look like the one in Hong Kong?

Some miscalculations cost more than others. But we never understand the error until it is too late.