While I was reading The Immeasurable World: Journeys in Desert Places, Donald Trump had yet to reverse his shameful policy of separating undocumented migrant children from their parents at America’s border. A picture was circulating on social media of two children lying in a cage in Nogales, Arizona, after being detained by the United States Border Patrol. The picture was presented as an example of the singular cruelty of the Trump administration – but it had been taken in 2014, during the presidency of Barack Obama.

Obama’s administration, and others before it, also detained immigrant families – parents and children together. Since the Nineties, as William Atkins points out, the doctrine of…