President Trump’s recent threat to escalate his trade skirmish with China into a full-scale trade war is a foolish gambit with little historical precedent. It is also hard to take seriously, given how quickly Mr. Trump changes his mind and how rarely and clumsily he tends to follow through on tough talk.

Mr. Trump said on Thursday that he wants to slap tariffs on an additional $100 billion in Chinese imports in response to Beijing’s plan to retaliate against an earlier American proposal that was aimed at $50 billion in Chinese goods. The president also said he was seeking ways to protect American farmers hurt by Chinese retaliation — a move that could result in fresh trade fights with other countries as they seek to defend their farmers from subsidized American grain.

If you’re confused or shocked by these announcements, you are not alone. Most experts say that a trade war between the world’s two largest economies would hurt American businesses, farmers and workers whose profits and livelihoods depend in part on commerce with China. That’s probably why the Standard & Poor’s 500-stock index fell more than 2 percent on Friday.

Historians say there is little precedent for Mr. Trump’s direct and forceful targeting of China. The last time a president used trade policy to hurt specific nations was when Thomas Jefferson and Congress restricted trade with Britain and France in the early 1800s, said Douglas Irwin, an economics professor at Dartmouth College who recently published a history of American trade policy. Those trade battles ended up leading to the War of 1812. “We haven’t seen anything like this in centuries,” he said.