Mr. Moon, who has said he wants to try to resolve the North’s nuclear crisis through dialogue, has also suggested that South Korea must “learn to say no” to Washington. He has already signaled a softening stance toward North Korea by encouraging aid groups to visit the country, although the North has rejected those offers since Seoul supported new United Nations sanctions.
Analysts said that as protesters demonstrated against the Thaad installation and South Korean businesses pressured the government to improve relations with China, Mr. Moon may have decided that suspending the progress of the missile defense system was politically expedient.
“I think he is trying to find a diplomatic way to slow down the process to placate the business community and placate his political supporters,” said Stephen R. Nagy, senior associate professor of politics and international studies at International Christian University in Tokyo.
Mr. Moon may also have sensed that China was not going to back down. When Lee Hae-chan, South Korea’s special presidential envoy, visited Beijing last month, President Xi Jinping did not concede anything during a meeting they jointly oversaw.
China’s strategy is to stand firm in its objections to Thaad to force Mr. Moon to modify — or even eliminate — a missile defense system that the Chinese suspect he does not like, either.
The defense system officially went into operation late last month on an abandoned golf course in Seongju, 135 miles southeast of Seoul, when two of six launchers were installed. United States military officials have said that the system is already “operational and has the ability to intercept North Korean missiles.”
This week, Mr. Moon accused the Ministry of Defense of trying to dodge a full environmental assessment as required by the law.
According to the law, any military installation on a site of more than 330,000 square meters requires a full analysis of the potential environmental and social effects. The ministry had divided the site, at 700,000 square meters, into two parcels to expedite the installation.
Supporters of Mr. Moon said the president was simply working to ensure that the Thaad battery complied with the law.
“The previous administration wasn’t really clear and transparent about the review process, and basically this is a legal procedure,” said Choi Jong-kun, a professor of political science at Yonsei University in Seoul.
Mr. Choi said that the president was eager to follow the legal procedure given that he was elected after his predecessor, Park Geun-hye, was impeached and ousted after accusations of corruption.
“The previous government failed to defend the constitutionality of the legal process in many fields,” Mr. Choi said. “So this president cannot repeat those same mistakes.” He added: “Is he saying ‘no’ to the United States? No. He is saying ‘yes’ to his constitutional responsibility.”
A spokeswoman for the United States forces in South Korea referred requests for comment to the Blue House.
Opponents of Mr. Moon said the suspension was probably a first step toward rejecting the missile defense system altogether. Oh Shin-hwan, a spokesman for the conservative-leaning Bareun Party, said in a statement that because the environmental review would take more than a year to complete, “the government does not intend to deploy the remaining four launchers.”
“North Korean provocations are occurring almost every day,” the statement continued. “And South Korea is saying that it will defend the country with half of the Thaad system. It is in effect saying that the government will not take into consideration the safety of Korean citizens, United States service members and their families.”
Analysts said it was too early to determine the ultimate outcome of the assessment. The early deployment “was rushed, so if the rush has been slowed down a bit, it’s not the end of the world,” said Gordon Flake, the chief executive of the Perth-U.S. Asia Center at the University of Western Australia.
But, he added, Mr. Moon “has to be aware of a fundamentally changed strategic environment in the last several years in Northeast Asia.” As North Korea rapidly develops the capability to launch missiles that could hit Japan and American bases in the region, Mr. Flake said, “decisions that South Korea makes have regional and global implications.”
Scott A. Snyder, senior fellow for Korea studies and director of the program on United States-Korea policy at the Council on Foreign Relations, said in an email that American officials should explain the need for the defense system to the new administration and that Mr. Moon’s supporters should not reject it simply because Ms. Park had approved it or give in to pressure from China.
“Thaad is at risk of becoming overpoliticized,” Mr. Snyder wrote. “And both sides need to take a deep breath and reaffirm common objectives and means for dealing with them rather than allowing Thaad to become a neuralgic and reflexive object of confrontation.”
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