There was a grain or two of truth in what Donald Trump said last week about migration. A polity is indeed vulnerable to being eaten from the inside by the entry of large numbers of people from very different backgrounds who are not inclined to embrace its culture in preference to their own. But the same does not go for his further advice that the best way to stop the rot was by bringing in the military.

For one thing, there would be legal difficulties. By international treaty every country has to require its vessels to do what they can to rescue those in danger of being lost at sea. This applies even to those cynically abusing the rule by engaging, like many Channel migrants, in moral blackmail by deliberately endangering themselves and then demanding to be picked up. There is also grave

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