On Wednesday, U.S. and Mexican officials will take up a subject that a week ago they had little reason to think they would face. Then, President Trump overruled advisers and announced plans for imposing new tariffs on Mexico. He views the tariffs as leverage to get the Mexican government to crack down on unauthorized immigration. Many of those migrants, in pursuit of asylum, make their way to the American border, their numbers having increased sharply in recent months.

That surge has overwhelmed the system for processing asylum requests. So there is much reason for action, and even for meeting with Mexican officials to address ways the countries can work together. Do the tariffs, set initially at 5 percent on all Mexican goods and reaching 25 percent by October, promise to help?

It is hard to see how. No doubt, Mexican officials will remind their American counterparts that they already have turned around more than 600,000 migrants from Central America the past five years. They also have accepted many, and they recently have launched efforts to bolster security with the aim of stopping migrants from entering their country.

Actually, Mexico has the same problem, and its immigration system also is overwhelmed. Why then does the president think Mexico can do better on its own when his administration admits it needs assistance? He has demanded what Mexico hardly can deliver — accepting all asylum seekers to ensure they do not get to the American border.

This migrant challenge is an opening for collaboration between the two countries. That translates to such things as additional resources for Mexico in handling migrants. It calls for attending closely to why people flee Guatemala, Honduras and El Salvador. They want to escape the poverty, violence, corruption and a lack of economic opportunity. Unfortunately, the Trump White House has moved away from needed aid programs.

By working together, Mexico and the United States acknowledge how interdependent they have become, especially their economies. Which goes to another concern about the tariffs, the harmful economic fallout, something readily cited by the U.S. Chamber of Commerce. The rising levy would result in higher prices for American consumers and businesses. Efficient supply chains, built during the past 25 years, would be disrupted.

As their economy suffered, especially at the highest tariff level, Mexicans would likely head north in higher numbers, the border problem getting worse.

The president might respond reassuringly that Mexico will fold to his demands. Yet, here, too, are troubling consequences. The president’s tariff announcement has arrived just as Canada, Mexico and the United States are moving toward approval of the updated trade agreement they struck last fall. That deal is all about removing trade barriers or staying clear of tariffs. In departing from such thinking, the president diminishes further his credibility, inviting the questions about his reliability, and that of the country, as a negotiating partner.

The erosion of trust and needless uncertainty are bad for diplomacy and business.

Consider also that the president has used as legal cover a 1977 law permitting tariffs when the country faces a national security emergency. What’s happening at the border is a humanitarian test, not a matter of the country’s overall safety. U.S. Sen. Rob Portman, a Cincinnati Republican, and others in his majority caucus have pushed back against such presidential overreaching involving steel and aluminum products, and potentially cars. It is good to see warnings from Republicans on Capitol Hill this week about the president going too far with his latest tariff threat. Will that result in firm defiance, even a veto-proof majority in the end? This misguided policy deserves as much.