SEOUL — President Biden is repudiating the legacy of President Trump in a swing through America’s Northeast Asian allies in which one message is indelibly clear: No more love-ins with North Korea’s leader, Kim Jong-un, and no more backsliding on America’s commitment to defense of the region. 

Mr. Biden did not mention America’s troubled relationship during Mr. Trump’s presidency with South Korea’s president at the time, Moon Jae-in, as he wound up 48 hours in Korea. He and South Korea’s newly elected president, Yoon Suk-yeol, reaffirmed the need for “strategic deterrence” in a visit Sunday to the Korean Air and Space Center, the primary command post if North Korea attacked the South.

Messrs. Biden and Yoon descended into the underground bunker at Osan Air Base, 40 miles south of Seoul, before Mr. Biden chatted with American troops and took off for Japan. “Deterring threats and underwriting stability,” Mr. Biden said, is “vital today for not only the [Korean] peninsula but for the world.”

Mr. Yoon, showing Mr. Biden around the center, was still more emphatic. Eager to demonstrate his hardline policy toward North Korea and his desire to reverse the appeasement policy of his predecessor, Mr. Yoon said Mr. Biden’s visit “symbolizes the strong security alliance between South Korea and the United States.” The center, he said, was the “central control body” for destroying the North Korean regime if war broke out.

The two presidents cemented their relationship one day after Mr Yoon in his summit here with Mr. Biden moved decisively and explicitly to restore the joint military exercises that Mr. Trump abruptly canceled after his summit with Mr. Kim in Singapore nearly four years ago.

That reversal was clear in a joint statement the two presidents issued just after their summit in which they agreed “to initiate discussions to expand the scope and scale of combined military exercises and training on and around the Korean Peninsula.”

Those words came as a relief to both American and Korean military commanders who had long said their forces had to work together on the ground, in the air, and at sea to be sure of their ability to withstand attack by North Korea.

Mr. Trump took his military chiefs, notably Secretary of Defense Mattis, by surprise when he announced he was canceling war games scheduled right after his summit with Mr. Kim in June 2018. Military officers have often said the war games played on computers since Singapore were not enough to be sure military forces from both countries could work effectively together.

While analysts said North Korea is preparing for another long-range missile test and its seventh underground nuclear test, Messrs. Biden and Yoon in their joint statement talked tough about preparations to counter whatever Mr. Kim has in mind.

The two leaders “reaffirm the commitment of the U.S.to deploy strategic U.S. military assets in a timely and coordinated manner as necessary,” they said. They also promised “to enhance such measures and identify new or additional steps to reinforce deterrence in the face of DPRK destabilizing activities.”

The understanding reached by Messrs. Biden and Yoon presented a challenge to North Korea, whose formal name is the Democratic People’s Republic of Korea, at a time when Mr. Kim is battling the spread of Covid among impoverished North Koreans. Mr. Biden said in a press conference after the summit that Washington has offered aid to North Korea, as has Seoul, but that the North had not responded.

Mr. Biden did not mention Mr. Trump by name but made clear his intention of undoing the attempts of the previous administration to appease the North. Asked if he would consider meeting Mr. Kim, he said he would, but qualified that remark by saying Mr. Kim would have to show he was “sincere” and “serious.”

On Sunday, asked if he had a message for Mr. Kim, Mr. Biden said it would be, “Hello. Period.”

That curt response made a mockery of Mr. Trump’s claim to have “fallen in love” with Mr. Kim at their Singapore summit, where they signed a joint statement promising to work for “denuclearization” of the Korean peninsula. Mr. Trump saw Mr. Kim two more times — in Hanoi in February 2019 and finally in an impromptu rendezvous at the Demilitarized Zone between the two Koreas in June 2019.

The Hanoi summit broke up when Mr. Kim refused Mr. Trump’s demand for denuclearization, but they sought unsuccessfully to come to terms in their meeting in June when Mr. Kim came down to the DMZ to see Mr. Trump, who was in South Korea to see South Korea’s Mr. Moon. In 2018, Mr. Moon saw Mr. Kim three times, but Mr. Kim ignored his pleas for another meeting after the failure of the Hanoi summit.

The conservative Mr. Yoon during his campaign for president promised to improve relations with Washington after the strains caused during the presidency of the liberal Mr. Moon, who had wanted Washington to go along with a joint statement declaring that the Korean War, in which active fighting was halted by an armed truce in July 1953, was really over.

That statement was viewed as a step on the way to a peace treaty, which would have required Washington to withdraw its troops from South Korea and remove sanctions imposed on North Korea after its nuclear and missile tests. North Korea conducted its sixth, most recent nuclear test in September 2017, but Mr. Kim has ordered a series of missile tests, including that of an intercontinental ballistic missile in March.

On paper, at least, Messrs. Biden and Yoon appeared to have reversed the policies of both their predecessors. In their joint statement they reiterated “their common goal of the complete denuclearization of the Korean peninsula” and agreed to further strengthen the airtight coordination to this end.” They said they shared the view that North Korea’s nuclear program “presents a grave threat not only to peace and stability on the Korean peninsula but also the rest of Asia and the world.”

That kind of language represented an escalation, rhetorically, from the tone of Mr. Moon’s comments, though Mr. Moon clearly supported the America-Korea alliance and increased spending to modernize South Korea’s armed forces.

Mr. Biden, on his first visit to the region as president, will go on reversing Mr. Trump’s policies in Tokyo in his summit Monday with Prime Minister Kishida. On Tuesday he will seek to fortify America’s commitment to Asia in a meeting with fellow leaders of “the quad,” a grouping of the United States, Japan, India, and Australia banded in quasi-alliance with obvious overtones against Communist China.

The highlight of Mr. Biden’s visit to Japan will be the unveiling of the Indo-Pacific Economic Framework. It pledges nations to work together on free trade and market-opening. The framework is a successor to the Trans-Pacific Partnership that President Obama saw as vital to American interests. Mr. Trump withdrew Washington from the pact on his first day in office in 2017.

Like Mr. Biden’s understanding with Mr. Yoon on defense, the wording on the economic framework is likely to appear vague, and its success will depend on the ability to turn nice-sounding words into meaningful action.

Similarly, economic concerns were an important component of Mr. Biden’s meeting with Mr. Yoon. In response to worries of a break in the flow of semiconductors and other vital electronic products, they agreed on the need for “secure, sustainable, and resilient global supply chains” and promised to build on “international cooperation fostered by the U.S.-led summit on global supply chain resilience.”

Mr. Biden met the leaders of Korea’s two biggest chaebol, or conglomerates. Samsung chief Lee Jae-yong on Friday showed him around Samsung Electronics’ huge chip factory near Osan, and Hyundai Motor’s chairman, Chung Eui-sun, met Mr. Biden on Sunday. Samsung is investing $17 billion in a new chip plant in Texas, and Hyundai Motor announced plans to invest $5.5 billion in a plant in Georgia for producing electrically powered vehicles. Mr. Chung also told Mr. Biden that Hyundai would invest another $5 billion in research and development with U.S. companies “in diverse technologies, such as robotics, urban air mobility, autonomous driving and artificial intelligence.”