Didn’t the president of the United States, Donald John “Smart Person” Trump learn a thing while getting his economics degree from the University of Pennsylvania?

Someone surely must have taught those econ students about the consequences of trade wars, of how badly many of those conflicts can go. If so, then what was Donald Trump doing when his prof offered that counsel? Was he asleep? Was he skipping class that day, spending his time chasing women and grabbing them in their private parts?

Trump reportedly got so out of sorts that he announced a decision to impose a 25 percent tariff on imported steel and a 10 percent tariff on imported aluminum. Then he said that winning a trade would be “so easy.”

So easy? Is he out of what passes for his mind?

Trump has declared economic war on our closest allies. They are, oh let’s see, Canada, Mexico, Germany and Great Britain. Yet he seeks to punish China because, according to the president, they have stolen jobs from U.S. steelmakers.

He now is making mainstream Republican officeholders — those who adhere to the party’s policy of free trade and its opposition to protectionism — queasy in the extreme.

Trump’s decision has sent the stock market into a frenzy of unpredictability.

He thinks he knows what he is doing. Analysts who actually do know something about international economics and its impact on geopolitics have a different view.

They say the president doesn’t know a damn thing. He is acting out of pique. He doesn’t listen to the advice of economic advisers he has gathered around him — folks like the Treasury secretary and the head of his Government Economics Council — who oppose this tariff nonsense.

Hey, he told us in the summer of 2016 that “I, alone” can repair what he thinks ails the country. No, Mr. President. You, alone are making a shambles of our economic alliances.