The steel and aluminum tariffs are ostensibly aimed at punishing China, which has been driving down prices for those commodities by producing far more metal than the world can use. But Mr. Trump’s move will have a limited effect on China because much of the steel and aluminum the United States imports actually comes from allies like Canada, Brazil, South Korea and Mexico. Further, the move could hurt American businesses that use these metals, including auto and construction companies, which will now pay more for a critical raw material.

If Mr. Trump were truly interested in getting China to reduce its excess production, he would have worked with the European Union, Canada, Japan, South Korea and other countries to put pressure on Beijing. Those nations tend to be closely aligned with the United States and have also been hurt by China’s mercantilist economic policies.

But rather than make common cause with them, Mr. Trump has angered American allies with this move. Top officials in Canada and the European Union are already threatening to retaliate forcefully against the new Trump tariffs. They could do so by bringing cases against the United States at the World Trade Organization, or by doing what Mr. Trump did and unilaterally imposing new tariffs on American exports like soybeans and Boeing planes. As it happens, Canada imports more American Steel than it exports to the United States, giving it the ability to hurt the very industry Mr. Trump claims to want to help. The country’s foreign minister, Chrystia Freeland, says Canada buys about half of American steel exports.

It’s also not clear that these tariffs will help Mr. Trump achieve his goal of increasing manufacturing employment in the United States. For example, businesses located in this country already produce about 70 percent of the steel used here. Experts say for every new job at a steel mill or aluminum smelter that is created by this trade decision, the country could lose as many or more jobs at businesses that use those metals, which will now cost more. This is one of the reasons that trade wars are, in fact, not easy to win.