Photo/IllutrationDefense Minister Itsunori Onodera, right, and South Korean National Defense Minister Song Young-moo address the Asia Security Summit in Singapore. (Gakushi Fujiwara)

Japanese officials were sent scrambling after U.S. President Donald Trump announced a halt to joint military exercises with South Korea, a move that could radically change the security situation for Japan and the region.

“U.S. troops based in South Korea as well as U.S.-South Korea military exercises play a very important role for national security in East Asia,” Defense Minister Itsunori Onodera told reporters on June 13. “While North Korea may have pledged to denuclearize, it has not yet made any specific moves toward that goal, so we have no intention of changing our current surveillance structure.”

Trump said after his June 12 summit in Singapore with North Korean leader Kim Jong Un that such military exercises would be suspended while denuclearization negotiations were continuing between the two nations.

Trump also touched upon the possibility of reducing the number of U.S. troops based in South Korea.

Top officials in Japan were quick to express concerns over what would constitute a major shift in U.S. defense policy.

Masahisa Sato, a state minister for foreign affairs who once served as an officer in the Self-Defense Forces, said on a TV program on June 12 that the ramifications from Trump’s words could be enormous for Japan.

“To be honest, I was stunned,” Sato said. “It would have a huge effect on Japan’s national security and the role of the SDF.”

In preparations for the Trump-Kim summit, Japan dispatched Shotaro Yachi, head of the secretariat for the National Security Council, to Singapore.

According to sources, Yachi met with John Bolton, Trump’s national security adviser, and received assurances that the topic of U.S. troops based in South Korea would not be brought up during the summit.

The relief felt by Japanese officials from Bolton’s assurances was soon blown away by Trump’s comments during and after his meeting with Kim.

A suspension of joint U.S.-South Korea military exercises would not only affect the combat readiness of U.S. troops in South Korea but would also affect joint training involving the SDF.

In particular, Japan joined the United States and South Korea for joint training exercises from June 2016 mainly to track ballistic missiles launched by North Korea and share that intelligence.

Since China has also long criticized the joint U.S.-South Korea military exercises, the military balance in East Asia could change if Washington were to suspend the war games.

Japan has also been nervous about any U.S. move to reduce troop strength in South Korea.

At the Asia Security Summit, also known as the International Institute for Strategic Studies’ Shangri-La Dialogue, held in Singapore 10 days before the Trump-Kim summit, U.S. Defense Secretary Jim Mattis said in a speech that the issue of reducing the number of U.S. troops in South Korea could become a topic for discussion between Washington and Seoul if the threat level was reduced.

The day after the speech, Onodera met with Mattis in Singapore and asked about the possibility of reducing the U.S. troop level in South Korea. Mattis said the United States currently had no such plans.

However, if positive negotiations between the United States and North Korea increases that possibility, the effects on Japan would be unavoidable.

“The military balance in the Asia-Pacific region would change if China shifted its troops now deployed to deal with a potential military conflict involving North Korea to other areas,” a high-ranking Foreign Ministry official said.

(This article was written by Nanae Kurashige in Singapore and Senior Staff Writer Taketsugu Sato in Tokyo.)