Other examples of deterrence have fared no better.
Britain's government concluded from the Brexit referendum that
it should redouble efforts to create a "hostile environment" for immigrants.
It ended up sending notices to people who had arrived in Britain from the Caribbean in the 1950s,
ordering them to produce documents to prove they were British.
The harassment, detention and deportations that followed resulted in the resignation of the home secretary.
Likewise, in 2015 European governments argued that rescuing boats carrying migrants from north Africa merely encouraged more to risk that journey.
Then as many as 1,200 people drowned in ten days, and Europeans were horrified at the cruelty being meted out in their name.
European leaders concluded that voters were not pro-drowning after all.
The left often concludes from this that people calling for enforcement are cruel and racist.
But that is wrong, too.
In principle countries must be able to secure their borders and uphold the law.
In practice a policy of neglect invites a backlash
that helps people like Matteo Salvini, leader of the Northern League (slogan: "Italians First"),
or Horst Seehofer, Germany's interior minister, who has threatened to bring down Angela Merkel.
The outrage feeds on itself.
Mr Salvini wants to deport hundreds of thousands of migrants from Italy;
Mr Seehofer wants to send tens of thousands of migrants to Italy.
The Platonic ideal of an immigration policy is one that has the consent of the host country.
It treats migrants humanely but also firmly,
swiftly returning those who arrived illegally or whose claims to asylum have failed.
This is easy to prescribe but hard to enact.
Courts are overstretched, many cases are hard to adjudicate and poor countries may not want their citizens back.
And so rich countries tend to pay poorer ones to set up vast holding-pens for humans,
as Italy does with Libya and the EU does with Turkey.
This involves something which would not be tolerated at home,
but is somehow acceptable because it is out of sight.
Europeans were right to condemn the separation of children.
But they face a wave of migrants from their populous, poor, war-torn neighbours.
When they draw up their own policies, they should remember their discomfort this week.