SEOUL—The jeep appeared out of nowhere on the North Korean side and sped southward toward the heavily guarded border. Its driver, dressed in the dark green uniform worn by junior enlisted North Korean soldiers, seemed to be fleeing from his comrades.

U.S. and South Korean troops monitoring security cameras on Monday afternoon watched as the vehicle hit a ditch and became stuck within eyesight of the line that separates the South from Kim Jong Un’s dictatorship.

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