Not only will this do nothing to reduce steel and aluminum capacity in China, it will more than likely prompt the European Union, Canada and Mexico to retaliate by imposing new tariffs on American products, hurting businesses and workers across the country. The president is also effectively isolating the United States from its closest allies — the very countries it needs to work with to put pressure on China to change its economic policies.

Even the Aluminum Association, which represents most American-based producers of the metal, said it was “disappointed” by Mr. Trump’s decision. “Today’s action does little to address the China challenge while potentially alienating allies” and disrupting supplies of aluminum and raw materials that American producers need, the group’s president and chief executive, Heidi Brock, said in a statement.

Indeed, why would Europe, Canada and Mexico, which also suffer from Chinese overproduction of steel and aluminum, have any incentive to work with an administration that seems to care so little about the consequences of its actions on their economies and workers?

Mr. Trump’s bullying appears to be pushing voters in at least one country toward more extremist leaders. In Mexico, Mr. Trump’s combative attitude appears to be helping the leftist presidential candidate Andrés Manuel López Obrador ahead of a July 1 election. Many experts fear that Mr. López Obrador has authoritarian tendencies and could undermine democracy in Mexico if he becomes president.

The tariffs may be Mr. Trump’s way to demonstrate that he will still punish countries for cheating the United States. They arrive on the heels of lawmakers’ criticism that the president has gone easy on the Chinese telecommunications company ZTE. Even after the company flagrantly violated American sanctions against the export of advanced technology to Iran and North Korea and was identified as a security risk by American intelligence agencies, the Trump administration said last week that it would let ZTE continue buying American semiconductors and other components. That agreement, which the president said was meant to protect Chinese jobs, appears even more suspicious in light of the fact that it came shortly after China awarded trademarks to Mr. Trump’s daughter Ivanka. It also came after a Chinese state-owned company struck a deal to build a theme park with an Indonesian business next to a hotel and golf course that the Indonesian company is building with the Trump Organization.