President Trump’s approach to migration, which has reshaped American border enforcement with its “zero tolerance” policy, follows a model pioneered by the European Union and Australia — though they may not have pursued it with Mr. Trump’s bombast.

Like Mr. Trump’s policies, defined by child-filled detention camps and his extraordinary move to ban nearly all asylum claims at the southern border, this model relies on two strategies to keep migrants and refugees from reaching the border at all:

1) Make the journey so daunting that they will not even attempt it.

2) Enlist poorer countries to detain or expel those who do anyway.

That approach, which Europe and Australia have taken to extremes beyond many of Mr. Trump’s policies, was meant to curb record migrant arrivals and the white backlash to them that was upending Western politics. Those arrivals have since declined, and populist revolts cooled.

But the lessons of Europe and Australia’s experience may not be so straightforward.

Strategies to deter or block migrants, research has found, may temporarily reduce arrivals. Over the long term, however, they may simply push migrants to try even more dangerous routes. They may also end up requiring governments to take ever more extreme measures to shut down each new round of arrivals.