Tensions have risen in the region after Kim Jong-un, the North Korean leader, proclaimed during his New Year’s Day address that his military was preparing to conduct its first test launch of an intercontinental ballistic missile.
But Asian nations have also been concerned about the conflicting signals from President Trump, which has made reassuring allies the first order of business for Mr. Mattis.
During his campaign, Mr. Trump mused that the United States could save money if nations like South Korea and Japan developed their own nuclear weapons — comments that ran counter to decades of American nonproliferation policy.
Mr. Trump said on Twitter last month that North Korea would be stopped from developing the capability to reach the United States with a nuclear weapon. But he did not say whether he was referring to taking military or diplomatic steps. “It won’t happen,” he tersely declared of a North Korean missile test.
One of Mr. Trump’s first acts as president was to formally withdraw from the Trans-Pacific Partnership agreement, which had been an important pillar of the Obama administration’s trade policy in the region. Critics say the withdrawal by the United States will give China an opportunity to expand its influence.
More recently, Mr. Trump reassured Prime Minister Shinzo Abe of Japan in a phone call on Saturday of the United States’ “ironclad” commitment to his nation’s security, according to a statement from the White House. He made the same assurance on Monday in a call with the acting president of South Korea, Hwang Kyo-ahn.
The various messages — some spontaneous, some premeditated — have made Mr. Mattis’s otherwise traditional statements of support for South Korea and Japan a message with strategic importance.
“It is a priority for President Trump’s administration to pay attention to the northwest Pacific,” Mr. Mattis said. “I am going to get current by listening to them, finding out where their issues are and then we are going to work together and strengthen our alliance.”
In South Korea, Mr. Mattis’s choice to visit it and Japan for his first official trip abroad was seen as a reflection of the new administration’s seriousness in dealing with North Korea’s growing nuclear threat. His trip was also reassuring for South Koreans, who have become anxious over Mr. Trump’s accusations during his campaign that their nation and Japan were getting a free ride in their national defense by not paying enough for the American military presence.
“In less than two weeks since taking office, President Trump has been almost daily issuing executive orders upending not only his predecessor Obama’s policies, but also some of the longstanding American policy lines,” said Go Myong-hyun, an analyst at the Asan Institute for Policy Studies in Seoul, the capital. “In this context, Secretary Mattis’s trip, which serves to reaffirm the United States’ commitment to its allies, is an unexpected present.”
The fraught political situation in South Korea — President Park Geun-hye faces an impeachment trial — also poses a challenge, given that a presidential election may be held as early as the spring.
“Mattis is going to meet with people who probably aren’t going to be in office in a few months,“ said Joel S. Wit, an expert on Korea at the School of Advanced International Studies at Johns Hopkins University.
Mr. Mattis will meet with an array of officials, including Mr. Hwang.
One crucial political issue is the Thaad antimissile defense system, which is designed to intercept missiles like the North Korean Rodong. The United States and South Korea initially said they wanted to deploy the Thaad system by the end of the year. Given North Korea’s bellicose behavior, there has been some speculation that the air defense could be deployed sooner.
In his comments to reporters, Mr. Mattis made the case for deploying the system, without detailing how quickly that might happen.
“Thaad is for defense of our allies’ people, of our troops who are committed to their defense,” he said. “Were it not for the provocative behavior of North Korea we would have no need for Thaad out here. There is no other nation that needs to be concerned about Thaad.”
But key opposition leaders mindful of the possible election oppose the deployment, arguing that it will do little to defend South Korea from the North’s plentiful short-range missiles while angering China, which could engage in economic retaliation if the system is deployed.
The Chinese have long objected to the deployment of limited missile defenses, fearing it could be the first step toward a more comprehensive antimissile shield that would have the capability to defend against Beijing’s own nuclear deterrent.
Mr. Hwang has said that the Thaad deployment is “inevitable” given the North’s rapidly growing missile threat, adding that his government was consulting with the United States to deploy the Thaad system “as soon as possible.”
“Thaad is a defense tool whose deployment should not be delayed any more,” he said at a recent news conference. “We are explaining our position in various ways to neighboring countries like China who are concerned about the Thaad deployment.”
Moon Jae-in, an opposition leader who is considered the front-runner among potential presidential candidates, has argued that South Korea should use the Thaad program as diplomatic leverage with China, keeping open the possibility that it would not be deployed if China helps rein in North Korea’s nuclear and missile programs. He has said that the Thaad deployment would help create tension between the United States and China and make South Korea’s diplomatic position more complicated.
“Given our standoff with North Korea and its nuclear program, our security and the alliance with the United States are our top priority,” Mr. Moon told reporters recently. “But the best scenario for us is when the U.S. and China get along well. If there is friction between the two, it’s not going to be easy for us.”
Mr. Mattis is not planning a trip to the demilitarized zone between the two Koreas. Nor is he planning to meet with South Korea’s political opposition. Mr. Mattis will meet with top American military leaders but is not scheduled to visit with American troops
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