A 5 percent tariff on Mexican goods would notionally amount to about $17 billion on U.S. imports from Mexico, touching everything from industrial components to fruit and crude oil. In reality, it is difficult to say how much money would be raised, because buyers respond to tariffs in unpredictable ways. In any case, many of those costs will be borne by American consumers and — this cannot be emphasized enough — American businesses that rely in some part on imported inputs. More important, it would cause uncertainty around a North American supply chain that has evolved organically over many years as the result of enormous investment by American companies and their business partners.

President Trump envisions a tariff that will potentially ratchet up to 25 percent.

The president here is unnecessarily complicating his own life. He has just overseen the successful renegotiation of NAFTA, which will be reconstituted as the U.S.–Mexico–Canada Agreement (USMCA). But that agreement has not yet been ratified — not by the United States, and not by Mexico. Imposing punitive tariffs over a policy dispute unrelated to trade five minutes after negotiating a new trade pact makes the Trump administration — and the United States — look like an unreliable negotiating partner. Mexico is not wrong to resent it, and even Trump allies such as Senator Chuck Grassley (R., Iowa) are against him on this.