SEOUL, South Korea — Japanese Prime Minister Shigeru Ishiba announced his resignation Sunday, clearing the way for a leadership contest within his ruling, but wobbly, Liberal Democratic Party.

The development indicates America’s leading Asian ally has re-entered a period that bedeviled it between the 1990s and early 2000s: “Revolving door” leadership. Mr. Ishiba is Japan’s fourth prime minister — all from the LDP — in five years.

Mr. Ishiba, who told a press conference of his “deep regret,” said he was resigning to prevent “a decisive split” in the LDP.

Though his public approval ratings have bounced upward in recent weeks, he oversaw three election disasters in which the LDP lost its majorities in both houses of the Diet, as well as the Tokyo Prefectural Assembly.

Seen as reform-minded in areas such as increasing immigration, he was unpopular with party conservatives.

In office, he mastered a key crisis. In July, his negotiating team reached a deal with the Trump administration for 15% tariffs on Japanese exports, packaged with pledges of $550 billion in investments in the U.S.

The deal was finalized with the signing of an executive order by Mr. Trump on Thursday. After that, opponents within the party unsheathed their knives.

Last week saw high-profile resignations from Mr. Ishiba’s cabinet, damaging statements from party kingmakers, and threats by party rebels to force an intra-party leadership race as early as Monday.

Speaking to The Washington Times on Friday, Cambridge University Professor of Modern Japanese Politics and the International Relations of East Asia John Nilsson-Wright predicted Sunday’s development.

“Ishiba’s initial defense of his decision to stay on as PM was framed in terms of his need to oversee difficult trade talks with the U.S., but with these now formally over, this argument has less cogency,” he said. “The cratering of internal party support makes it inconceivable that Ishiba can remain in post.”

Mr. Ishiba will stay on as prime minister and LDP president until the outcome of the intra-party leadership race. A date for that has not been set, but it is expected imminently.

With the center-right LDP facing an upstart hard-right rival — the Shinseito Party, which, inspired by MAGA, proclaims “Japanese First” as its motto — some expect a swing to the LDP’s right.

A likely contender is the party’s leading hardcore conservative, Takaichi Sanae. Ms. Sanae, a former minister of Economic Security, is seen as the heir to Japan’s longest-serving prime minister, Shinzo Abe, who was murdered in 2022.

However, Mr. Nilsson-Wright warns, “It remains an open question whether her hawkish conservative leanings will complicate the newly improved ties with South Korea, or if she will moderate her traditional views in favor of a more pragmatic style of foreign policy.”

Seen as a China hawk, she favors revising Japan’s peacetime constitution to give its armed forces more leeway. Domestically, she is anti-immigration and anti-same-sex marriage.

Another likely contender is Agriculture Minister Shinjiro Koizumi, son of former LDP Premier Junichiro Koizumi. Mr. Nilsson-Wrights considers him “ … youthful and telegenic and likely to have widespread public appeal, but there are doubts in some quarters about his policy expertise and ability to lead the party.”

Regardless of who takes over, he or she will need to be able to compromise with opposition parties, given the LDP’s loss of majority governance.

Meanwhile, Japan’s national challenges are multifaceted.

At home, the country is struggling with rising inflation, a creeping demographic crisis and a sense of “Japan Passing” — the idea that the country’s best days are behind it.

Mr. Ishiba’s successor will need to navigate the constitutionally pacifist island nation through an increasingly hazardous geopolitical minefield.

In its own backyard, Japan faces off against an array of hostile nuclear powers: Russia, North Korea and China.

Strategically sited, Japan’s southern islands command critical naval choke points northeast of Taiwan.

Japan’s strongest security shield is the U.S. It is America’s leading ally in the Indo-Pacific, and host to more GIs than any other country outside the U.S. itself.

However, Mr. Ishiba has balked at demands from Washington for a more than doubling of current defense spending, raising questions over how secure the bilateral alliance is.

• Andrew Salmon can be reached at asalmon@washingtontimes.com.