BY NOW it is clear that no big new stretch of physical wall will rise anytime soon along the Southwest border. Owing to President Trump, however, a towering wall of hostility directed at Mexico is already surely in place. And its effects, while impermanent, are nonetheless real.

Those effects are already imposing stiff costs on the United States in prestige, goodwill and moral authority, as well as more tangible expenses: International travel associations, as well as U.S. hotel executives, warn that rising nationalism, stoked by Mr. Trump, is likely to constrain the flow of tourists into the United States this year. Given that travel and tourism contribute more than 8 percent of America’s gross domestic product, amounting to some $1.5 trillion annually, the unintended business losses could be stiff.

The more direct, and intended, impact of Mr. Trump’s fiery rhetoric has been a dramatic drop in illegal border crossings, as measured by apprehensions by Border Patrol officers along the frontier with Mexico. While crossings and apprehensions during the Obama administration’s second term were already at their lowest level in four decades, the level since Mr. Trump took office has plummeted further. In March, the number of illegal crossers apprehended — just under 12,200 — fell 64 percent from the same month last year.

The likely main cause of that precipitous drop is the rhetoric Mr. Trump has aimed at undocumented immigrants, reinforced by media coverage of actual and threatened deportation sweeps, and expectations that the administration will assemble the bigger and more aggressive deportation force that the president promised in his campaign. For Central Americans who would enter the United States illegally, the perceived heightened risk of detention and removal has shifted their cost-benefit calculus — especially given that smugglers demand rates as high as $10,000.

Stanching the flow of illegal immigration at the border is a good thing, as long as it lasts, notwithstanding the irony that it undercuts Mr. Trump’s own argument for a big wall. There is little dispute that the United States, like any country, is entitled to control its borders and demand that those who enter do so legally. How to treat unauthorized immigrants who have lived here for years, and now form part of the United States’ fabric, is a different question.

Mr. Trump’s success in jawboning down the flow of illegal border crossings relies on the fear he has inspired, which is likely to dissipate unless his words are backed up by sustained action, including expanded capacity in detention centers and immigration courts. The danger is that the collateral damage stemming from that fear — in ill will from the United States’ neighbors and the wider world, in addition to divided, distrustful communities at home — may outlast the temporary benefits derived from Mr. Trump’s bombast.