Donald Trump’s saber rattling may or may not deter Kim Jong Un, but it’s had an effect south of the border.

In the first few months of this year, illegal border crossings have dropped precipitously. It is an early proof of concept that yes, it is possible to secure the border and a victory, even if a provisional and incomplete one, for President Trump’s enforcement agenda.

Once you stripped away the impossibilities from Trump’s rhetoric on immigration during the campaign — there wasn’t going to be a wall along the entire border paid for by Mexico, nor were there going to be mass deportations and a Muslim ban — the core of his message was a commitment to crack down on illegal border crossings.

This is happening. It has been reported in the press, but it almost never makes it into the conversation about his first 100 days.

If Trump had promised to almost immediately reduce illegal border crossings from Mexico to a 17-year low, it would have been dismissed as bombast. But here we are. On the border, there is cause to be, if not tired of, at least encouraged by all the winning.

According to the Department of Homeland Security, there were 17,000 arrests at the border in March, the lowest figure since 2000, and down significantly from the nearly 60,000 arrests in December. The evidence on the ground backs up the statistics. A week or so ago, the Los Angeles Times reported that it found a “once-bustling crossing point” along the Rio Grande “desolate.” The message has been received that President Trump is, to coin a phrase, a bad hombre. His tough rhetoric alone would be enough to make would-be migrants think twice about coming across the border. But the administration also has tightened up on enforcement; both immigration arrests and so-called detainers — requests to local law enforcement to turn over to the feds illegal immigrants in jail — have increased.

The fees charged by so-called coyotes to bring people across the border have risen, reflecting the higher risk. A jump from, say, $3,500 to $8,000 represents an enormous new expense for a mother in Honduras, especially if her chances of staying in the United States are diminishing.

The early trend at the border is a rebuke to the fatalists who have argued that it’s impossible to reduce illegal immigration because it is the product of ineluctable economic forces. It is certainly true that jobs in the U.S. are a magnet. But the cost-benefit calculation people make when considering to come here illegally or not is obviously subject to change based on changing incentives.

Mexico and its southern neighbors haven’t suddenly become better countries over the past several months. Nor has the U.S. jobs market become less alluring — indeed, it might be getting stronger. Enhanced enforcement, real and perceived, is clearly affecting the decision-making of would-be migrants.

Some caveats. First, the Trump administration will have to continue to strengthen enforcement. Otherwise, illegal immigration will bounce back to its normal trend. The experience after the 1986 amnesty confirms this. The law was supposed to include tougher enforcement, and this expectation initially suppressed immigrant flows. When it became clear that it was all talk, illegal immigration continued as before.

Second, visa overstays are a large and perhaps growing contributor to illegal immigration. This deserves as much attention as the border. Finally, the administration will have to resist the urge to declare victory and go home. The moderates in the White House may be tempted to soften Trump’s image via a grand bargain on immigration, arguing that it’s “mission accomplished” on enforcement and time to pivot to a large-scale amnesty.

Despite the hostile press coverage, what Trump has done so far on immigration is hardly radical and basically represents a return to the pre-Barack Obama status quo, when the federal government didn’t openly flout its own laws. For now, it has gotten results.

Rich Lowry can be reached via e-mail: comments.lowry@nationalreview.com.