President Trump climbed down on separating families at the border, but the underlying argument isn’t going away.

The central question at the border isn’t whether we should separate families — even most hard-liners in the Trump administration would prefer to hold families together — but whether migrants should stay in the United States or not.

Trump’s hopes to salvage his “zero tolerance” policy of prosecuting all illegal border-crossers via his executive order ending family separation, although the practical and legal obstacles will be substantial. The reaction among immigration advocates has gone from outrage about family separations to consternation about family detention, because their ultimate goal is to let the migrants come into the United States and stay.

This is not to deny that the first iteration of the “zero tolerance” policy was a fiasco. The justifications for the policy from administration officials were different and often clashing, and the effort to pin the whole thing on the Democrats was wildly unconvincing.

Fact is Democrats, in particular, didn’t give us any of the rules that have made closing the border to Central American migrants impossible. The Flores consent decree, which makes it difficult to hold kids longer than about 20 days, grew out of a court case 20 years ago. An anti-trafficking law that prevents us from quickly returning home Central American kids — because they are from noncontiguous countries — was a bipartisan measure signed into law by George W. Bush.

What is true is that the law makes it impossible to hold Central American parents and children together for any length of time. The children have to be released, and if you are going to keep them together with their parents, the parents have to be released, too. This is the forcing mechanism for waving Central American migrants into the country — more than a quarter of a million children and members of a family group over the past two and a half years — and Trump is right that Democrats have no interest in changing it.

When Republicans this week proposed fixes to remove these perversities in the law and to expedite the asylum process and provide for more detention space, Chuck Schumer had no interest. He thought he could back down Trump unilaterally from the family separations — correctly, as it turned out — and Democrats have no interest in making it easier for Trump to remove anyone from the country.

It’s easy to lose sight of the radicalism of this position. It’s understandable to oppose deporting an illegal immigrant who has been here for, say, 10 years. But these migrants are illegal immigrants who, in some cases, literally showed up yesterday.

The question they pose isn’t whether we are going to let illegal immigrants who are already here stay but whether we are constantly going to welcome more, in a perpetual, rolling amnesty. It is, in short, whether we have a border or whether a certain class of migrants can — for no good reason — present themselves to the authorities and expect to be admitted into the country.

Some of these migrants will claim asylum, but these claims are mostly bogus. There’s no doubt that they are desperate, and desperate to get into the United States. But they aren’t persecuted back home, even if they fear gangs or a violent boyfriend.

The merits don’t matter under the current system, though. If an asylum-seeker passes a credible fear interview — almost all do — and comes into the United States pending adjudication of his case, it’s unlikely that he’ll ever be seen again. And why not? Who wouldn’t take advantage of that opportunity?

Trump is right to want to end this dynamic and swiftly and reliably deport new migrants, which would be the only sure deterrent against the ongoing influx. Now, he wants to hold families together. Unless Congress acts, he’s going to run up against the same rules he’s been complaining about, and he will still get political opposition, although on changed grounds.

Increasingly for the left, the true enemy is enforcement, and the battle has just been joined.