The one great service of Donald Trump’s extended peregrinations on immigration policy is to have demonstrated how, in the end, there’s only one place to go.

You can rail for a year about the squishy soft, weak-kneed and stupid politicians who have opened our borders to the wretched refuse of Mexico. You can promise to round them up — the refuse, that is, not the politicians (they’re next) — and deport them. And that may win you a plurality of Republican primary votes.

But eventually you have to let it go. For all his incendiary language and clanging contradictions, Trump did exactly that in Phoenix on Wednesday. His “deportation task force” will be hunting … criminal aliens. Isn’t that the enforcement priority of President Obama, heretofore excoriated as the ultimate immigration patsy?

And what happens to the noncriminal illegal immigrants? On that, Trump punted. Their “appropriate disposition” will be considered “in several years when we have … ended illegal immigration for good.” Everyone knows what that means: One way or another, they will be allowed to stay.

Trump’s retreat points the way to the only serious solution: enforcement plus legalization. The required enforcement measures are well known — from a national E-Verify system that makes it just about impossible to work if you are here illegally, to intensified border patrol and high-tech tracking.

The one provision that, thanks to Trump, gets the most attention is a border wall. It’s hard to understand the opposition. It’s the most venerable and reliable way to keep people out. The triple fence outside San Diego led to a 90 percent reduction in infiltration. Israel’s border fence with the West Bank has produced a similar decline in terror attacks into Israel.

The main objection is symbolic. Walls, we are told, denote prisons. But only if they are built to keep people in, not if they are for keeping outsiders out. City walls, going back to Jericho, are there for protection. Even holier-than-thou Europeans have conceded the point as one country after another — Hungary, Macedonia, Bulgaria, Austria, Greece, Spain, why even Norway — has started building border fences to stem the tide of Middle Eastern refugees.

The other part of the immigration bargain is legalization. What do you do with the 11 million already here? In theory, you could do nothing. The problem ultimately solves itself as the generation of the desert — those who crossed the border originally — is eventually replaced by its American-born children who are automatically legal and landed.

But formal legalization is a political necessity. It gets buy-in from Democrats who for whatever reason — self-styled humanitarianism or bare-knuckled partisanship — have no interest in real border enforcement. Legalization is the quid pro quo. If they want to bring the immigrants “out of the shadows,” they must endorse serious enforcement.

Such a grand bargain could and would command a vast national consensus. The American public will accept today’s illegal immigrants if it is convinced that this will be the last such cohort.

This was the premise of the 1986 Reagan amnesty. It legalized almost 3 million immigrants. Because it never enforced the border, however, three has become 11.

And that’s why the Gang of Eight failed. They too got the sequencing wrong. The left insisted on legalization first. The Gang’s Republicans ultimately acquiesced because they figured, correctly, this was the best deal they could get in an era of Democratic control.

The problem is that legalization is essentially irreversible and would have gone into effect on Day One. Enforcement was a mere promise.

Hence the emerging Republican consensus, now that Trump has abandoned mass deportation: a heavy and detailed concentration on enforcement, leaving the question of what happens to those already here either unspoken (Trump on Wednesday) or to be treated “case by case” (Trump last week).

The Trump detour into — and retreat from — deportation has proved salutary. Even the blustering tough guy had to dismiss it with “we’re not looking to hurt people.”

The ultimate national consensus, however, lies one step further down the road. Why leave legalization for some future discussion? Get it done. Once the river of illegal immigration has been demonstrably and securely reduced to a trickle, the country will readily exercise its natural magnanimity and legalize.

So why not agree now? Say it and sign it. To get, you have to give. That’s the art of the deal, is it not?

© 2016, The Washington Post Writers Group