Now, as North Korea carries out a series of provocative missile and nuclear bomb tests, that alliance is straining at a time when both nations may need it more than ever.
President Trump issued a blast of antagonistic comments in the last few days that have made South Koreans doubt that they can take the alliance for granted any longer.
On Twitter on Thursday, he declared that “talking is not the answer!” in dealing with North Korea, casting aside the push by the new South Korean president, Moon Jae-in, to hold talks with the North. On Saturday, he threatened to withdraw the United States from a five-year-old free trade agreement with South Korea over what he considers its unfair protectionistpolicies. And on Sunday, after North Korea detonated its most powerful nuclear device yet, he essentially called the South Koreans appeasers.
”South Korea is finding, as I have told them, that their talk of appeasement with North Korea will not work, they only understand one thing!” Mr. Trump said on Twitter.
The tone of Mr. Trump’s statements stunned officials here and underscored what unlikely partners he and Mr. Moon are, at a time when their countries’ 67-year-old military alliance faces an ever-more-dangerous regime in Pyongyang.
Mr. Moon, who was elected in May promising to seek dialogue with North Korea, fired back at Mr. Trump, insisting that the crisis be resolved peacefully.
”We can never tolerate another catastrophic war on this land,” his office said in a statement Sunday evening. “We will not give up our goal of working together with allies to seek a peaceful denuclearization of the Korean Peninsula.”
Mr. Moon has supported Mr. Trump’s push for tougher sanctions against North Korea, and in a call on Monday, their first since the nuclear test on Sunday, the two leaders agreed to lift the weight limit on South Korean conventional warheads, Park Soo-hyun, a spokesman for Mr. Moon, said. Removing the 500-kilogram restriction, part of a treaty with the United States aimed at preventing a regional arms race, could give the South greater power to strike the North in the event of military conflict.
Mr. Moon and Mr. Trump also agreed to “push for maximum pressure and sanctions against North Korea” at the United Nations Security Council, Mr. Park said.
But Mr. Moon has argued that sanctions and pressure alone have failed to stop the North’s advances in nuclear and missile technology. And while Mr. Trump has threatened North Korea with “fire and fury,” Mr. Moon has said there must be a peaceful solution because South Koreans, not Americans, would bear the brunt of war.
He called the North Korean nuclear test on Sunday “disappointing and infuriating.”
Lee Seong-hyon, an analyst at the Sejong Institute near Seoul, said: “Trump considers Moon naïve for insisting on dialogue with North Korea when it keeps conducting missile and nuclear tests. Trump is asking Moon, ‘Are you with us or not?’”
Mr. Trump’s threat to cancel the trade agreement squeezes South Korea on its economy, which is already suffering in part because of South Korean cooperation with the United States.
After North Korea tested its first intercontinental ballistic missile in July, Mr. Moon responded by authorizing an American missile-defense system that the Americans had been pushing for and that he had resisted. But authorizing the missile defense system angered China, which retaliated by cutting back on South Korean consumer products, forcing the South Korean carmaker Hyundai to temporarily shut down assembly lines in China.
Mr. Trump is not the first American leader to be skeptical of a progressive South Korean president’s approach to the North. Since North Korea was first discovered to be developing nuclear weapons in the 1990s, the allies have not always been on the same page.
Washington feared that liberal South Korean presidents’ preference for dialogue and openness were helping North Korea buy time and secure funds for its nuclear weapons program. South Korean progressives argued that dialogue with the North slowed down its nuclear program and even halted it, at least for a while.
Mr. Moon was a close ally of President Roh Moo-hyun, a liberal who championed the so-called sunshine policy of engaging North Korea with dialogue, trade and aid shipments. Mr. Roh owed his 2002 election in part to a wave of anti-American sentiment, spurred by the deaths of two South Korean schoolgirls who were run over by an American military vehicle.
”Washington still suspects that President Moon is Roh Moo-hyun 2.0,” said Kim Ji-yoon, an analyst at the Asan Institute for Policy Studies in Seoul.
Moreover, Mr. Moon and Mr. Trump lack personal chemistry, analysts said, inhabiting opposing political spheres and coming from vastly different backgrounds.
Mr. Moon was a former human rights lawyer defending labor activists and political dissidents while Mr. Trump was a former real-estate magnate.
While Mr. Trump called for America First, Mr. Moon said during his campaign that he would “say no to the Americans” if necessary, in contrast to the previous, conservative government, which he portrayed as in lock step with the United States.
If Mr. Trump sees North Korea primarily as a nuclear threat, Mr. Moon, a son of Korean War refugees, tries to resolve it in a broader context of building reconciliation and eventually achieving reunification on the divided Korean Peninsula.
During his campaign, Mr. Moon alarmed Washington by saying that he would review Seoul’s agreement on the deployment of the American missile-defense system known as Thaad. The system has infuriated China, which said the powerful American radar on its doorstep undermined its own security.
Mr. Moon has since reversed himself, but Washington had a taste of his ability to say no when he took a swipe at Mr. Trump in a nationally televised speech last month. “No one should be allowed to decide on a military action on the Korean Peninsula without South Korean agreement,” he said.
Kim Keun-sik, a political-science professor at the University of North Korean Studies in Seoul, said: “Moon Jae-in had an uneasy start with Washington. The unease kept building up.”
The North never showed an interest in talks, further undermining Mr. Moon’s position.
“The latest nuclear test is a wake-up call for those in South Korea who still believe that dialogue is possible with North Korea,” said Lee Byong-chul, a senior fellow at the Institute for Peace and Cooperation in Seoul. “Kim Jong-un will never give up his nuclear weapons.”
The challenge for South Korea now is how to live with the reality of a nuclear-armed North. South Koreans fear that once North Korea acquires nuclear-tipped ICBMs, it will use them to drive a wedge between Washington and Seoul by offering to freeze its nuclear program in return for the withdrawal of American troops from the South.
In the worst-case scenario, they say, North Korea may attack the South and use its nuclear arsenal to deter the United States from intervening.
Mr. Moon also faces domestic pressure from the right, with South Korean conservatives clamoring for tactical nuclear weapons, and accusing Mr. Moon of endangering the alliance with Washington while “begging North Korea for dialogue.”
“Moon Jae-in became president four months ago, and the 50 million South Koreans have already become hostage to the North Korean nukes,” Hong Joon-pyo, leader of the conservative opposition Korea Liberty Party, said on Monday.