Mr. Trump, who visited with American and South Korean troops at Camp Humphreys south of Seoul, noted that the United States military had positioned three aircraft carriers and a nuclear submarine in the Pacific.
“We have many things happening that we hope, we hope — in fact, I’ll go a step further — we hope to God we never have to use,” Mr. Trump said.
When pressed by a reporter, Mr. Trump declined to say whether he still thought negotiations with North Korea would be a waste of time, making an uncharacteristic effort to avoid a remark that might have inflamed tensions.
“I don’t want to say that — I just don’t want to say that,” Mr. Trump said. “You can understand.”
His visit to Seoul was the most diplomatically challenging leg of Mr. Trump’s 12-day, five-country trip through Asia, bringing him face to face with a public and a president wary of his combative approach on North Korea. To many of Mr. Moon’s progressive supporters, Mr. Trump poses as much of a threat to peace as Mr. Kim, if not more so.
“Don’t come, Trump! You talk about war whenever you open your mouth,” a large banner read during a protest near the U.S. Embassy on Tuesday. “Go away, Trump!” hundreds of labor activists and other progressives shouted in downtown Seoul, where thousands of police were deployed to keep security. “No Trump, no war!”
A short distance away, across a police blockade, hundreds of conservatives welcomed Mr. Trump with South Korean and American flags. South Korean conservatives are deeply skeptical of Mr. Moon’s approach, calling it naïve. They back Mr. Trump’s hawkish view of the North, although they, too, stop short of supporting war on the Korean Peninsula.
“We believe in Trump!” their signs read.
Mr. Trump’s visit to South Korea appeared choreographed to avoid potentially provocative moments. Unlike many previous presidents, he eschewed the traditional visit to the Demilitarized Zone separating the North and South, where past American commanders in chief have peered into the North through binoculars, projecting a menacing mien.
To Understand North Korea, Look to Its Mountains
And while his advisers have signaled that Mr. Trump may soon return North Korea to the State Department’s list of state sponsors of terrorism, he did not address that issue on Tuesday.
Mr. Trump’s change in tone notwithstanding, his advisers have increasingly been making the case that North Korea’s ambition is to reunify the Korean Peninsula by force, and that traditional deterrence — fear of overwhelming retaliation — cannot stop the North once it has a nuclear weapon capable of hitting the United States.
Many observers are skeptical of both assertions, especially in South Korea, which would suffer greatly if a pre-emptive strike on the North led to all-out war. They argue that the leaders of Pyongyang’s Communist regime are not planning to use nuclear weapons to attack the United States or its allies, because they know that Washington would overwhelmingly counterattack. Rather, they say, North Korea is building its nuclear weapons out of fear, hoping that the arms will protect the country from invasion — or from outside intervention, in the case of a domestic uprising — allowing the North to focus on economic development.
In addition, all of the countries around North Korea, even its old allies China and Russia, have said they would oppose war on the Korean Peninsula.
“The view that North Korea would start war to communize Korea doesn’t make sense anymore,” said Kim Yong-hyun, a professor of North Korean studies at Dongguk University in Seoul. “North Korea knows that if it ever uses a nuclear weapon, it means self-destruction.”
South Korean officials said they hoped that Mr. Trump’s visit to this densely populated capital of 10 million people would bring home to him the consequences of a potential war. Mr. Moon supports Mr. Trump’s call for “maximum” sanctions and pressure, but says that those alone will never persuade North Korea to give up nuclear weapons.
Mr. Moon has urged Washington to engage the North with dialogue and discuss a way out for Pyongyang. But so far, Mr. Trump has ignored such requests, even ridiculing Mr. Moon’s efforts as “appeasement.”
American officials in the region, too, were hoping the president’s trip would help Mr. Trump gain a new perspective on how Seoul and Tokyo view the threat from up close; the regional trends affecting Asia beyond North Korea, including Chinese assertiveness; and the costs and consequences of military action, according to one U.S. official.
From a dining hall at Camp Humphreys, where Mr. Trump had lunch with troops, to the Blue House, where he met with Mr. Moon, everything he saw on Tuesday could become a fiery battlefield in the event of a military exchange with North Korea.
But if the firsthand experience left an impression on Mr. Trump, he did not show it. At times, his tone was almost blithe. Before being briefed by American and South Korean military commanders, he said of the confrontation with the North, “Ultimately it will all work out. Because it always works out — has to work out.” General Vincent Brooks, the American commander, wore a grim expression as he listened to the president.
Mr. Trump also reprised his role as a chief salesman for the American defense industry. “South Korea will be ordering billions of dollars of that equipment, which, frankly, for them makes a lot of sense,” Mr. Trump said. “And for us, it means jobs; it means reducing our trade deficit with South Korea.”
On the eve of a visit to China, Mr. Trump also made a pointed plea for countries around the world to use their influence to bring North Korea to heel, saying, “It is unacceptable that nations would help to arm and finance this increasingly dangerous regime.”
But in a reminder of the challenge that Washington faces keeping even its allies on the same page, South Korea invited an 88-year-old Korean “comfort woman,” or a former sex slave for Japan’s World War II military, to Mr. Trump’s state banquet and put on the menu a dish of shrimp said to have been caught in waters near a set of disputed islets.
The sex slave issue is one of the most contentious dividing Tokyo and Seoul, and the islets — known as Dokdo in South Korea and Takeshima in Japan — lie at the center of a bitter territorial dispute. The Japanese government protested the banquet arrangements, accusing South Korea of “moves that could negatively affect the close coordination” among the allies.
As Mr. Trump arrived in Seoul, there was renewed evidence of activity at a site in North Korea where the regime has conducted underground nuclear tests. It was not clear, however, whether the activity suggested another imminent test or merely construction on a tunnel that could be used for tests.