In hindsight, we never should have expected the war on terror to unify the United States, as the two world wars or, to a large extent, the Cold War did.

Those conflicts pitted the United States against nation-states whose defeat — or, in the case of the Soviet Union, containment — could be intelligibly defined and realistically anticipated. Bright lines could be drawn between friend and foe. Rules of war could be more readily identified and upheld.

The war that began on 9/11 and continues today, in various forms, in Afghanistan, Iraq, Syria, Yemen — even parts of Africa — was and is a protracted, shadowy conflict against irregular forces. You might call it counterinsurgency on a global scale. By its nature, such a war forces difficult and divisive policy questions on any democratic government obliged to wage it.