Thus, what was supposed to be a calm holiday weekend dissolved into jitters over a dual threat: the specter of nuclear war on the Korean Peninsula, and the possibility of an economic standoff among world powers, as Mr. Trump considers walking away from a major trade agreement with South Korea as early as this week.
Administration officials said Mr. Trump’s attack-on-all-fronts approach was necessary to keep the pressure on adversaries and allies alike. But outside observers saw a danger in Mr. Trump’s efforts to fulfill a core populist campaign promise on trade even as he tried to use the issue as leverage on security matters.
“In a circumstance where we’re going to need close cooperation with not only South Korea but China as well, he’s coming out swinging at all of them rather than trying to build support and coordination,” said Ely Ratner, a top national security official in the Obama administration. “It just looks so haphazard.”
Still, Mr. Ratner said North Korea’s latest nuclear test could finally spur China, which views nuclear tests as far more serious than the North’s series of ballistic missile launches, to undertake a more serious crackdown on its neighbor.
“I think the nuclear test has a chance of pushing China into a place it’s never been before,” Mr. Ratner said.
In a salvo of Twitter messages over six hours on Sunday, Mr. Trump called North Korea’s biggest nuclear test to date “very hostile and dangerous.” In invoking South Korean “appeasement,” he criticized Seoul’s proposal to hold military talks with the North, saying of Pyongyang, “they only understand one thing” — meaning the threat of military force.
The Treasury secretary, Steven Mnuchin, said that he planned to draft a new sanctions package that would cut economic ties with anyone who did business with North Korea.
“There’s a lot we can do to cut them off economically, much more than we’ve done,” Mr. Mnuchin said, speaking on “Fox News Sunday.” He called Pyongyang’s actions “unacceptable” and stressed the need for stronger steps.
Mr. Trump’s threat to halt trade went much further, suggesting a move that would dramatically intensify the potential for conflict with China, which accounts for roughly 85 percent of all trade with the North.
Some critics dismissed the president’s suggestion as an empty threat, because, if carried out, it would most likely prompt an immediate…