But one way for China to respond is to weaken the renminbi and undermine the impact of those tariffs by making those products cheaper.

That’s why when China allowed its closely controlled renminbi to depreciate sharply against the dollar on Monday, it was taken as a sign that the trade war between the United States and China was getting worse.

The currency has since strengthened, easing this tension somewhat, but China isn’t the only trading partner the president has a problem with.

For instance, in June, after the European Central Bank said it might restart stimulus programs to bolster the economy, Mr. Trump accused it of pushing down the value of the euro, “making it unfairly easier for them to compete against the USA.”

“They have been getting away with this for years, along with China and others,” he said on Twitter.

A weaker dollar has other benefits. For instance, it could also bolster corporate earnings. Roughly 40 percent of the revenue of the biggest American companies now comes from overseas , and a weaker dollar means those foreign sales make a bigger contribution to the bottom line. Those higher earnings can help give the stock market a lift.

None of this is a secret. But in the past, governments have shied away from weakening their currencies, in part because they were afraid it would also lead to an ugly bout of inflation, which was traditionally viewed as the big risk of a weak currency. These days, inflation around the world is incredibly low and shows little sign of rising.