WASHINGTON, Aug. 20 -- South Korea's foreign minister learned that the United States was ending the Ulchi Freedom Shield exercises six days early the same way much of official Seoul did: by reading Donald Trump's post on Truth Social.

Foreign Minister Cho Hyun told reporters Thursday that "neither our government nor US officials knew what President Trump wrote on his Truth Social platform" before the announcement. There had been no advance notification, no bilateral consultation, no diplomatic forewarning of the kind that has historically accompanied any adjustment to the joint exercises that underpin the combined military posture on the Korean Peninsula.

The exercise itself, Ulchi Freedom Shield, had been scheduled to run from August 1...