What did Donald Trump do today?

He forgot how his first trade war went.

Today, Trump said that one of his first acts after being inaugurated would be to impose 25% tariffs on Mexico and Canada.

Specifically, he said that he would do this to force Mexico and Canada to prevent immigrants and drugs from coming across the U.S. border. Presumably he meant across the southern U.S. border, in which case it's not clear what Canada has to do with it.

As for Mexico, this isn't the first time that Trump has tried to outsource the United States' immigration policy to that country. In his first administration, his reliance on an Obama Administration agreement called Programa Frontera Sur, where Mexico would take steps to reduce the number of migrants passing through its southern border to the United States, gave Mexico enormous leverage over the United States.

Mexico is the United States' single biggest trade partner.

Tariffs are taxes paid by American consumers on foreign goods. As such, they tend to fall most heavily on low- and middle-class Americans, who cannot afford large price increases. In small amounts, they are a technical economic tool for working out trade imbalances or protecting domestic industries. 

Tariffs are not useful as instruments of foreign policy, because their function is to reduce trade that American consumers want. Because countries inevitably impose tariffs of their own on American goods in retaliation, the resulting trade war hurts all sides.

Trump had an opportunity to learn this lesson during his first term, when he entered into an absolutely disastrous trade war with China and the European Union. It cost hundreds of thousands of American jobs, slowed the growth of the U.S. economy to a crawl, and led to massive farm foreclosures because crop prices fell through the floor. There was a spike in suicides by bankrupted farmers as a result, after multiple taxpayer bailouts failed. It also spiked the overall U.S. world trade deficit—meaning that the United States imported more goods at higher prices for its consumers than before—which was the opposite of what Trump said it would do.

Trump also said today that he would impose huge tariffs on China, which, again, means imposing enormous taxes on Americans who want or need to buy Chinese goods, like computer hardware or prescription drugs.

Why does this matter?

• Presidents should be able to learn from catastrophic mistakes.

• Presidents should be able to admit catastrophic mistakes.

• The United States economy is too important for a president to be this ignorant about how it works.