YUKIHIRO SAKAGUCHI and SHUNSUKE SHIGETA, Nikkei staff writers

TOKYO — U.S. President Donald Trump lamented the impending loss of his “best friend” on the diplomatic stage during a call Monday with Japanese Prime Minister Shinzo Abe, who has announced his resignation due to health concerns.

“Shinzo will soon be recognized as the greatest Prime Minister in the history of Japan, whose relationship with the USA is the best it has ever been. Special man!” Trump tweeted after the call, which was requested by Washington.

Abe reportedly has been the world leader most trusted by Trump.

“There have been a lot of occasions where he was asked for his opinions by Trump or other leaders at international meetings,” an Abe aide said.

But this privileged position vanishes in late September after the selection of Abe’s successor, who will be thrust into the middle of an increasingly hostile relationship between Washington and Beijing.

The next prime minister’s top priority will be dealing with a China that has made rapid strides economically and militarily. Of particular concern are Beijing’s incursions into waters around the Japan-administered Senkaku Islands, which have been occurring with higher frequency and ever-larger vessels.

As Beijing grows into a greater threat, diplomacy with the U.S. becomes all the more crucial, given Washington’s importance to Japan’s security. Any frayed relations between the two allies would provide an opportunity that China or North Korea could exploit.

Tensions surged in the South China Sea last week after Beijing launched four missiles into an area where a U.S. reconnaissance plane reportedly flew earlier during Chinese military exercises. The missiles included the DF-26, dubbed the “Guam killer” for its ability to reach the American territory — a clear shot across Washington’s bow.

The launch reminded some observers of a 1996 incident in which China fired missiles near the Taiwan Strait in the run-up to the island’s first direct presidential election. At the time, Washington dispatched two aircraft carriers to the area, discouraging any further shows of force.

But “the situation is completely different from a quarter-century ago,” a senior official in Japan’s Foreign Ministry said. “Back then, there was an overwhelming difference in military power between the U.S. and China. Now China has a certain degree of confidence.”

China’s drive to build a “world-class” military on par with its American rival by around 2050 is shifting the balance of power in a region once tilted heavily toward the U.S. Beijing’s recent crackdown in Hong Kong has some in Washington fearing that a move on Taiwan or the Senkakus might be next.

One of the trickiest issues in the U.S.-Japan relationship is the question of Tokyo’s degree of responsibility for its own defense. The two sides are slated to hold talks this fall on Tokyo’s share of the cost for hosting American troops, as Trump has pressed Japan to contribute more.

Washington also is interested in Tokyo gaining the capability to strike enemy bases in response to an imminent attack, and the U.S. has raised the possibility of Japan hosting medium-range American missiles as well. But both proposals face difficulty in securing support from the Japanese public.

During his tenure, Abe pushed through security legislation at the cost of his approval rating. Whether his successor will be able to make similarly tough calls remains to be seen.

On the economic side, the U.S. and Japan are set to hold a new round of talks toward broadening the bilateral trade deal signed last year. Tokyo so far has avoided higher tariffs on its automobiles and parts, but the next prime minister may face such demands as Washington grows frustrated with the continued trade imbalance.

Most of the main candidates to succeed Abe have top-level defense or diplomatic experience, including current Foreign Minister Toshimitsu Motegi, former Foreign Minister Fumio Kishida, former Defense Minister Shigeru Ishiba and current Defense Minister Taro Kono, who previously served as foreign minister.

Chief Cabinet Secretary Yoshihide Suga, Abe’s right-hand man, has traveled to the U.S. in an official capacity, which is unusual for his position.

But diplomatic relationships like Abe’s are not built in a day. And the U.S. presidential election in November could put Trump rival Joe Biden in office, forcing a rewrite of Tokyo’s American strategy.

Additional reporting by Shotaro Miyasaka and Yuta Saito in Tokyo.