SEOUL, South Korea — For decades, the United States and South Korea have regularly staged military drills to prepare their soldiers to work together smoothly in the event of a crisis or conflict with North Korea.

But this year’s Ulchi Freedom Shield exercise, one of the two main drills conducted by the allies annually, could be disrupted after U.S. President Donald Trump ordered his Pentagon chief to “substantially” scale it back just before its start Monday.

He cited what he said was a good relationship with North Korean leader Kim Jong Un and South Korea’s refusal to join the U.S. war against Iran. It was unclear what parts of the 11-day exercise were being cut and to what extent, but many experts say Trump’s move threatens to hurt combined U.S.-South Korean readiness and eventually weaken their decades-long alliance.

The Ulchi Freedom Shield, or UFS, is a computer-simulated tabletop exercise meant to hone the allies’ ability to deal with potential regional security threats like North Korean aggression. The two countries usually hold field training exercises during the tabletop exercise period.

South Korea’s military had earlier said this year’s exercise would be similar in scale to previous years, with the participation of about 18,000 South Korean soldiers. The U.S. military said it would incorporate lessons from recent conflicts such as North Korea’s battlefield experiences on the Russia-Ukraine front. Local media reported South Korea and the U.S. had planned 14 field training exercises.

The UFS, along with a similar joint war game in the spring, is a central pillar of the U.S. -South Korea alliance, which dates back to the 1950-53 Korean War. But it’s also a long-running source of tensions with North Korea, which has denounced the exercises as a rehearsal for war and responded with provocative missile tests.

Last Friday, North Korea’s Foreign Ministry accused the U.S. and South Korea of plotting more provocative drills this year and vowed to respond with “a new level of a deterrent.” The U.S. and South Korea have said their regular drills are defensive in nature.

In a social media post, Trump called the exercise costly, “with much of these costs paid for by the United States of America.” During his first term, Trump complained about the cost of drills with South Korea, suspending joint exercises in 2018 to advance now-stalled nuclear negotiations with North Korea.

Gen. Xavier Brunson, commander of about 28,500 U.S. troops stationed in South Korea, visited South Korean Defense Minister Ahn Gyu-back on Tuesday for talks likely focusing on the adjustment of this year’s drills, according to media reports. South Korea’s Defense Ministry said it couldn’t confirm the report.

Yang Uk, an analyst at Seoul’s Asan Institute for Policy Studies, said the U.S. will likely participate in the tabletop exercise but cancel “some costly field training that requires large-scale movements of troops.”

Yang said that a downsizing or suspension of military drills matters because commanders of the allies change periodically. He said a one-year suspension of drills would see commanders unfamiliar with procedures of those drills taking part when they resume.

Jung Chang Wook, head of the Korea Defense Study Forum think tank in Seoul, said Trump’s step could result in the U.S. downsizing other regular drills with South Korea, posing “a big, crippling blow” to their joint operation posture.

Lee Illwoo, an expert with the Korea Defense Network, said downsizing the exercise will likely delay the planned transfer to a system in which wartime operational control would be exercised by a binational command led by a South Korean general.

Under the current system, the U.S. military would command joint forces in the event of war.

The South Korean and U.S. militaries had earlier said the exercise would be an opportunity to support preparations for the transfer. South Korean President Lee Jae Myung on Tuesday stressed the need to complete the transition during his term that ends in 2030.

Many South Koreans view the transition to South Korean leadership of the combined forces in wartime to be a matter of national sovereignty, but others worry that could weaken the alliance.