SEOUL – President Biden arrived here Friday on a five-day mission to America’s two crucial Northeast Asian allies, South Korea and Japan, in direct response to two regional foes, a resurgent China and its North Korean protectorate, which is battling Covid-19 and hunger.

During a trip that was remarkably well-timed, both by design and by chance, Mr. Biden and South Korea’s president will be getting down to how to stand up to China when they hold their summit on Saturday, 11 days after Yoon Suk-yeol’s inauguration. He defeated his liberal rival by less than 1 percent of the votes in the March election.

In an open act of defiance against China before Mr. Biden takes off for Tokyo on Sunday, they’ll travel 40 miles south of Seoul to visit the Korean Air and Space Operations Center at Osan Air Base, home of the U.S. Seventh Air Force. They’re not portraying that visit as a threat, but it carries the implication that America and South Korea are ready to increase military cooperation against China and North Korea after a period during which Mr. Yoon’s liberal predecessor, Moon Jae-in, severely discouraged anything that might anger North Korea’s leader, Kim Jong-un.

Mr. Biden’s visit was planned before Mr. Yoon’s election victory, but the timing could not have been better from the viewpoint of asserting Washington’s concern about a region that seemed to have fallen off the radar screen while Mr. Biden focused on Russia’s invasion of Ukraine.

That North Korea, after denying the existence of disease within its borders, is now having to fight Covid-19 as it spreads throughout the country adds to the urgency of Mr. Biden’s summits here and in Tokyo. North Korea has turned a deaf ear to offers of aid from both South Korea and Japan while relying on its benefactor, China, the source of virtually all its oil and half its food, to ship in badly needed supplies to fight a disease for which the North is almost entirely unprepared. North Korea has still not indicated any interest in vaccinating its people — “vaccines” appears to be a taboo in state propaganda, which has never used the word.

Now analysts are wondering whether Mr. Kim, while portraying himself as a benevolent leader plunging into battle against the disease, will go ahead with what have appeared to be sure-fire preparations to test another intercontinental ballistic missile capable of hitting targets in the U.S., or perhaps order tests of short- or mid-range missiles capable of carrying warheads to Japan and South Korea.

It’s also still possible Mr. Kim will order an underground test of a nuclear warhead. North Korea tested an ICBM in March for the first time since November 2017 but has not tested a nuclear warhead since its sixth such test in September of that year. Analysts, however, have spotted preparations for both nuclear and ICBM tests, and the deputy director of South Korea’s presidential national security office, Kim Tae-hyo, said “an action plan for how South Korea and the United States will strengthen reliable and effective extended deterrence” would be at the top of the summit agenda.

Any such test, of course, would play into the hands of Messrs. Biden and Yoon as they pledge to build on relations between Seoul and Washington after strains during the presidency of Mr. Moon, who annoyed Washington by persisting in pursuing a formal declaration that the Korean War was over. For North Korea and China to go along with this idea, the Americans would have had to lift sanctions imposed against North Korea for its nuclear and missile tests.

“One of the important takeaways from the Biden-Yoon summit will be the message that Seoul and Washington are in tight alignment on their North Korea policy,” Victor Cha and Ellen Kim wrote in a paper issued by the Center for Strategic and International Studies in Washington. “This includes a return to a complete, verifiable, and irreversible denuclearization (CVID) of North Korea as a common goal, little tolerance for North Korea’s provocations, and close policy consultations.”

North Korea’s “military provocations and Covid-19 outbreak will become a key issue agenda during the summit,” they wrote. Mr. Biden, they said, “will reaffirm the strong U.S. security commitment to defend South Korea, including the U.S. extended nuclear umbrella.”

Defense, however, will not be the only topic for the summit. Mr. Biden on Friday visited a Samsung Electronics plant near the major American base at Camp Humphreys, right after getting off Air Force One at Osan Air Base. Mr. Biden during his visit stressed cooperation on technology as seen in the steady flow of semiconductors produced at the plant, as well as at a $17 billion facility that Samsung is building near Austin, Texas. America and Korea, he said, will gain a “competitive edge in the global economy” provided supply chains remain “resilient, reliable, and secure.”

Underlining the importance of the American market, the leaders of the Samsung, Hyundai Motor, SK, and LG conglomerates, known as chaebol, will be at a state dinner Saturday night after Mr. Biden’s summit with Mr. Yoon.

Then, most importantly, Mr. Biden in Tokyo Monday will unveil his plan for an Indo-Pacific Economic Framework, a successor to the Trans-Pacific Partnership from which President Trump withdrew in one of his first acts after his inauguration in January 2017.

After Mr. Biden has done talking with Prime Minister Kishida of Japan, he will join in a meeting of “The Quad” — including India’s prime minister, Narendra Modi, and Australia’s prime minister, the identity of whom won’t be known for sure until after parliamentary elections Saturday, after which the Australian parliament will elect the prime minister. The incumbent, conservative Scott Morrison, faces labor leader Anthony Albanese.

Washington from time to time has been urging if not pressuring South Korea to join the Quad, but the South has been cool on the idea — no need is seen to upset China, still angered by South Korea’s acceptance of a missile site for Terminal High Altitude Area Defense, 125 miles south of Seoul.