President Trump said in a middle-of-the-night social media post early Thursday that he would come after the European Union and Canada if they banded together to “do economic harm” to America, opening a new front in the unfolding trade war.

“If the European Union works with Canada in order to do economic harm to the USA, large scale Tariffs, far larger than currently planned, will be placed on them both in order to protect the best friend that each of those two countries has ever had!” Mr. Trump wrote.

Mr. Trump’s post creates a new problem for the European Union, which is already trying to respond to his tariffs on steel, aluminum, autos and potentially a broader array of goods and services.

The United States is by far Europe’s most important trading partner, and the prospect of worse trading conditions has left the European Union scrambling to negotiate. But the Trump administration has showed little appetite to strike a deal so far.

“In the end, as it is said, one hand cannot clap,” Maros Sefcovic, the trade commissioner for the European Union, has said.

That has left Europeans seeking to strike new alliances and deepen existing trading relationships. And concerns about President Trump’s shifting stance on military support have driven partners like the European Union and Canada closer together. Canada is already working toward providing industrial support for Europe’s rearmament push.