Mr. Trump’s decision on Thursday to impose sanctions on Chinese entities that do business with North Korea was interpreted by some as a shot across the bow not only to Beijing, but also to Mr. Moon, since it emphasizes pressure over diplomacy. And the president’s approval of a $1.4 billion weapons sale to Taiwan provoked a sulfurous reaction from the Chinese government.
Taken together, the measures signal that Mr. Trump has moved into an aggressive, unpredictable phase of his strategy for dealing with one of the world’s tensest regions. On trade, at least, the United States will now be at odds with its two key partners in confronting the rogue government in North Korea.
Mr. Trump’s new approach will be a test of whether he can pursue the protectionist themes of his presidential campaign while still cooperating with China and South Korea on security issues.
“Trump has opted for coercion over engagement with allies and adversaries, even when the adversary is pressuring the ally,” said Evan S. Medeiros, who served as senior director on Asia in the National Security Council during the Obama administration.
China has punished several South Korean firms in retaliation for Seoul’s decision to deploy an American antimissile system. The United States says the system is designed to deter missiles fired by the North, but China says its own security is also affected.
Given that South Korea is already under intense pressure from the Chinese, some analysts said the harshness of Mr. Trump’s critique of South Korea on trade had caught them by surprise.
During the Oval Office meeting with Mr. Moon, Mr. Trump invited his commerce secretary, Wilbur Ross, to recite a list of grievances. Mr. Ross cited barriers that he said cut down the number of American cars sold in South Korea. He also complained about Chinese steel, which he said was dumped in South Korea and used to manufacture pipes for oil fields, which are then shipped to the United States.
The United States ran a $27 billion trade deficit with South Korea in 2016, a modest decline from the previous year but more than double the amount in 2011, the year before the renegotiated trade agreement took effect. “There are a lot of very specific problems,” Mr. Ross said.
Mr. Trump blamed President Barack Obama for the pact, even though the deal was first negotiated by President George W. Bush in 2007 and renegotiated by Mr. Obama, which is essentially what Mr. Trump is proposing to do.
Mr. Trump’s aides said he also planned to push ahead with his trade campaign against China — freed up by his conclusion that China’s president, Xi Jinping, had fallen short in pressuring North Korea to curb its behavior. Mr. Trump had delayed introducing measures against China to encourage Mr. Xi to use his influence with North Korea’s leader, Kim Jong-un.
In the coming days, the White House plans to publish the results of an investigation of the steel industry, which could lead to tariffs and other measures against imports. South Korea is the second-largest supplier of steel to the United States, by dollar value, after Canada.
China is much further back, but administration officials say the country has a pernicious effect on the global steel market because of its excess capacity. Surplus steel from China is shipped to other countries and ends up in products that are exported to the United States.
During the meeting with Mr. Moon, Gary D. Cohn, the president’s chief economic adviser, confined his remarks to China’s “many predatory practices in how they deal with us.” He told Mr. Moon he would be interested to hear how South Korea deals with its neighbor.
It is not yet clear whether Mr. Trump has given up on his relationship with Mr. Xi after cultivating him assiduously over a two-day summit meeting at the president’s Florida estate, Mar-a-Lago, in April. Mr. Trump has avoided derogatory comments or tweets about Mr. Xi, even as his administration has made moves, like the Taiwan arms sale, that angered Beijing.
Tellingly, on Friday Mr. Trump did not refer to China as an ally in the push to curb North Korea’s nuclear efforts. He said only that “other regional powers and all responsible nations” should join the United States, South Korea and Japan in enforcing sanctions on the North Korean government.
“The era of strategic patience with the North Korean regime has failed,” he said. “Frankly, that patience is over.”
In the days leading up to his visit, Mr. Moon and his aides had tried to erase any signs of daylight between him and Mr. Trump. The South Korean foreign minister said the government would not reverse the deployment of the American antimissile system, which Mr. Moon has halted until an environmental assessment can be completed.
Though Mr. Moon spoke of using “both sanctions and dialogue,” he said he supported Mr. Trump’s pressure campaign. “The threats and the provocations by the North will be met with the stern response,” he said.
But Mr. Moon said nothing about the trade issues that Mr. Trump raised. Like other foreign leaders, he came armed with business deals, including a $25 billion agreement to buy American natural gas, which earned perfunctory praise from Mr. Trump before he launched his critique.
“I saw Moon moving toward alignment and taking issues off the table,” said Scott Snyder, the director of the program on United States-Korea policy at the Council on Foreign Relations. “In a way, the only issue he couldn’t take off the table is the one the president holds powerfully and viscerally — the idea that Korea is a free rider.”
Trump Takes More Aggressive Stance With US Friends and Foes in Asia – New York Times}