Yoon Suk Yeol won South Korea’s highest office in 2022 by a threadbare marginhellowin, the closest since his country abandoned military rule in the 1980s and began holding free presidential elections.

Just over two years later, Mr. Yoon’s brief declaration of martial law on Tuesday shocked South Koreans who had hoped that tumultuous era of military intervention was behind them. Thousands of protesters gathered in Seoul to call for his arrest. Their country, regarded as a model of cultural soft power and an Asian democratic stalwart, had suddenly taken a sharp turn in another direction.

But the events that led to Mr. Yoon’s stunning declaration on Tuesday — and his decision six hours later to lift the decree after Parliament voted to block it — were set in motion well before his razor-thin victory. They were a dramatic illustration of South Korea’s bitterly polarized politics and the deep societal discontent beneath the surface of its rising global might.

It all came to a head when Mr. Yoon, once a hard-charging prosecutor who investigated former presidents, found himself on the receiving end of a political onslaught by a galvanized opposition.

Victory, but no mandate

Mr. Yoon, a conservative leader, has never been popular in South Korea. He won election by a margin of only 0.8 percentage points. The vote, analysts said, was more a referendum on his liberal predecessor’s failures than an endorsement of Mr. Yoon.

The bitterness of the campaign was reflected in a statement by Mr. Yoon’s main opponent, Lee Jae-myung, who would go on to lead the opposition to the Yoon government in Parliament.

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