Trump’s reasoning seems to be that the three countries haven’t done enough to stem the exodus. Maybe so. What the president hasn’t explained is how depriving quasi-failed states of assistance will help them do better by us.

Then again, Trump’s approach to Central America is an excellent illustration of the ways in which “America First” fails America.

If the administration is not going to have a realistic strategy for defeating gangs in the Northern Triangle — whether out of an ideological aversion for nation-building, stinginess with foreign aid, or mere indifference — it will need a realistic strategy for dealing with the inevitable fallout. If it doesn’t have that, as we still don’t, we’ll have exactly the kind of crisis at the border that Trump endlessly bemoans and which he is now doing so much to worsen.

In other words, if we’re not going to solve or at least try to mitigate problems over there, we will have to confront them over here. Previous presidents have made similar mistakes: Barack Obama’s ill-judged military exit from Iraq in 2011 created the conditions in which the Islamic State metastasized three years later (forcing our return). But it’s difficult to think of any president as eager as Trump to cut off his nose to spite his own face — not that there’s much face left to save at this point.

There are better options. Bill Clinton and then George W. Bush invested some $10 billion in counterinsurgency and counternarcotics efforts to rescue Colombia from the grip of jungle guerrillas and drug lords. The plan was expensive, took a decade, involved the limited deployment of U.S. troops, and was widely mocked.

Yet it worked. Colombia is South America’s great turnaround story. And nobody today worries about a Colombian migration crisis.

It’s always possible that Trump knows all this — and rejects it precisely because it stands a reasonable chance of eventually fixing the very problem that was central to his election and on which he intends to campaign for the next 18 months. Demagogues need bugaboos, and MS-13 and other assorted Latin American gangsters are the perfect ones for him.

But whether he gets that or not, it behooves Americans to know that the crisis at our border has a source, and that Trump continues to inflame it. The answer isn’t a big beautiful wall. It’s a real foreign policy. We used to know how to craft one.

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