A few months before the United States invaded Iraq, in 2003, Donald Rumsfeld, the defense secretary at the time, was asked on a radio show how long the war would take. “Five days or five weeks or five months,” he replied. “It certainly isn’t going to last any longer than that.” When George W. Bush departed the White House more than five years later, there were nearly 136,000 US soldiers stationed in the country.

The number of troops has fallen since then, but Bush’s successors have failed to withdraw the United States from the region. Barack Obama campaigned on ending…