United States President Donald Trump has ordered the Pentagon to substantially reduce this year's joint military exercises with South Korea, in a statement published on Sunday, United States time, that gave Seoul's refusal to join the war against Iran as one of his reasons.

The exercise is Ulchi Freedom Shield. It began on Monday and runs 11 days to 27 August, with 18,000 South Korean soldiers taking part alongside American forces. Trump said it was too late to cancel it and directed Defense Secretary Pete Hegseth to cut it back instead.

“I recently asked the President of South Korea if they would like to join us in the Denuclearization of the Islamic Republic of Iran, and they said, 'No thanks!'" Trump wrote. He also said he was "not happy with the fact that the United States has, long ago, agreed to participate in Joint Military Exercises with South Korea", described North Korea as unthreatening and respectful, and cited his relationship with the North Korean leader, Kim Jong Un.”

South Korea's Ministry of National Defense said the exercises would proceed as previously notified and scheduled. As of Monday the ministry had not announced any change to the program.

Ulchi Freedom Shield is the larger of the two annual exercises run under the alliance and is built around the defence of South Korea against a North Korean attack. Trump has taken issue with the drills before, calling them expensive and provocative in 2018.

What is new is the reason attached. On this occasion the exercises were reduced not over cost or over North Korea, but as a consequence of a decision Seoul took about a separate war in a separate theatre. South Korea is a treaty ally of the United States, and its position on Iran was a refusal to send forces to a conflict in which it is not a party.

That link between a security guarantee and an ally's conduct on a separate question is the part of this with the widest reach. Australia holds its alliance with the United States under the ANZUS treaty and hosts American forces on rotation through Darwin.

The drills are running now. Trump did not say what a substantially reduced Ulchi Freedom Shield involves, how much of the program is affected, or whether the reduction applies to the exercises scheduled for next year, and neither government has published a revised program.