Japan, the United States and South Korea have steadily built on their security cooperation amid China's rise and North Korea's belligerence, but the future appears precarious with the unpredictable Donald Trump returning to the White House, and South Korean President Yoon Suk Yeol mired in a crisis that threatens his leadership.

Experts in international relations warn the United States will likely pull back from its involvement in East Asian security affairs during Trump's second term starting Jan. 20, with the 78-year-old Republican expected to recommit to his "America First" platform and his related skepticism of multilateralism.

A close alignment between Japan and South Korea serves as a key foundation for the development of trilateral ties involving the United States. But direct ties between the two U.S. allies could be challenged if conservative Yoon is ousted and a leftist leader with a hardline stance on Japan is elected to replace him.

"We will no longer be able to take U.S. involvement on a global basis for granted" after Trump's inauguration, said Seiko Mimaki, an associate professor at the Graduate School of Global Studies of Doshisha University.

During the four years under U.S. President Joe Biden, who succeeded Trump in 2021, the United States recommitted to multilateralism and pursued "minilateral" arrangements to address various challenges in the Indo-Pacific, such as through fortified U.S.-Japan-South Korea ties, the U.S.-Japan-Philippines triad and the Quad group made up of the United States, Japan, India and Australia.

As for the U.S.-Japan-South Korea partnership, Biden played a pivotal role in widening the scope of three-way cooperation from primarily focusing on North Korea to addressing other realms, such as economic security and emerging technologies.

In August 2023, Biden, then Japanese Prime Minister Fumio Kishida and Yoon gathered at the U.S. presidential retreat of Camp David for their countries' first standalone summit, declaring a "new era of trilateral partnership." Biden administration officials have also worked to "institutionalize" the trilateral framework ahead of Trump's return to the Oval Office.

But Mimaki, an expert on U.S. politics and diplomacy, doubted that momentum would be carried forward.

The United States is juggling "three different fronts" simultaneously, which many in the public feel is "too much," she said, citing Ukraine, where the Biden administration has offered massive military support since Russia's full-scale invasion began in February 2022, the conflict-plagued Middle East, and the intensifying competition with China in the military and economic spheres.

Noting that Americans have become "more inward-focused in terms of global security issues," Mimaki said she feels Japan-U.S.-South Korea ties are "very fragile" despite efforts to institutionalize the partnership.

Adding to the uncertainty is the political turmoil in South Korea triggered by Yoon's abrupt but short-lived martial law declaration in early December. He insisted his shock move was intended to curb "anti-state" activities by the country's majority-holding opposition party.

Yoon, whose presidential powers were suspended after the parliament passed an impeachment vote on Dec. 14, awaits a decision of the country's Constitutional Court on his removal from office. The court has 180 days to decide.

Tadashi Kimiya, a professor with expertise in South Korean politics working at the University of Tokyo's Graduate School of Arts and Sciences, said it is "highly likely" that Yoon will be replaced, and in that case, the presidential election will probably be held no earlier than April even if the court delivers its verdict in two months.

"Common sense says (what Yoon did is) considered an abuse of power, to crack down on the opposition and self-destruct the country's order," he said.

Lee Jae Myung, leader of South Korea's main opposition Democratic Party, is "decisively holding the party in his grip" and therefore widely viewed as the leading candidate to be the next president for now, he added.

But a possible change of government in South Korea could mean a policy "flip" in dealing with Japan, as opposition parties have criticized Yoon's position on wartime issues as too conciliatory, Kimiya said.

When Yoon's predecessor Moon Jae In was in power, Tokyo-Seoul relations sunk to their lowest point in decades over issues stemming from Japan's 1910-1945 colonial rule of the Korean Peninsula. Disputes on trade and defense also heightened tensions.

Signs of a thaw emerged after Yoon became president in May 2022 and pledged to take a future-oriented approach toward the relationship with Japan. Yoon has also favored stronger ties with the United States.

Yoon rolled out a solution to a conflict with Japan over wartime labor compensation in March 2023, which had been a major sticking point, setting the stage for improvement in bilateral ties as well as the deepening trilateral ties with the United States.

The move, however, was an "obvious concession to Japan," Kimiya said.

Trump's return, meanwhile, casts a shadow over the Japan-U.S. alliance and highlights his belief that nations hosting U.S. troops do not pay enough for their protection, even though Japan has already set a target of doubling its defense budget to about 2 percent of its gross domestic product in five years through fiscal 2027.

Trump has named Elbridge Colby, a China hawk, as the next undersecretary of defense for policy. He is known for advocating that Japan should immediately expand its defense budget to about 3 percent of GDP.

Trump could also test ties between Japan and South Korea, Kimiya warned, noting that he may try to "pit" the two allies against one another to compete on which is more loyal to the United States. Trump would likely hope such a situation would allow him to negotiate with both nations to pay more U.S. troop hosting costs.

"To hold bargaining powers against the Trump administration, Japan and South Korea need to consult with each other on how to negotiate with Mr. Trump so that they will not be forced to adjust to his pace," the professor said.