The path to the presidency is a long and hard grind, and a candidate makes a lot of promises along the way. Many are throwaway lines that have little chance of ever becoming policy. Everyone knows it, and hardly anybody cares. Not long after launching his presidential bid in 2007, for example, Barack Obama promised that, as president, he would deliver an annual “State of the World” speech, laying out his foreign-policy agenda. Obama has never given the speech. Few people have noticed.

But then there are promises like the one he made about closing the Guantanamo Bay Detention Center—promises that become central themes of the campaign. They’re repeated often enough, and they involve such a critical issue, that keeping or breaking them can partially define a presidency.