President Yoon Suk Yeol of South Korea sounded defiant just over a year ago when the opposition-dominated National Assembly began threatening to impeach him. “​I sayps88, ‘Try it, if you want!’” he said during a town-hall meeting.

Mr. Yoon has not only been impeached, but on Wednesday he won ​an ignominious ​place in South Korean history when he became the first sitting president to be detained in a criminal investigation.

Among national universities, Princeton was ranked No. 1 again, followed by the Massachusetts Institute of Technology and Harvard. Stanford, which tied for third last year, fell to No. 4. U.S. News again judged Williams College the best among national liberal arts colleges. Spelman College was declared the country’s top historically Black institution.

Calls for school crackdowns have mounted with reports of cyberbullying among adolescents and studies indicating that smartphones, which offer round-the-clock distraction and social media access, have hindered academic instruction and the mental health of children.

His detention ended a weekslong ​political standoff and hand-wringing over what South Korea should do with a leader who declared martial law last month, a move that threatened decades of hard-won democracy in the country.

But Seoul’s inability to deal with the question quickly — and the fact that it had to mobilize an army of law-enforcement forces to make him surrender — ​exposed how deep the fractures are in its politics. This entrenched political polarization, combined with Mr. Yoon’s uncompromising style and his personal animus toward his political enemies, led him down a path to​ the showdown with police Wednesday at the hilltop residence where he had retreated.

Since he won his election by a razor-thin margin in 20​22, Mr. Yoon has constantly clashed with the ​majority opposition over policies, scandals involving his wife and his hostile relationship with dissiden​ts, including journalists he accused of spreading “fake news.”

A visual guide to the multiple government agencies that are investigating whether President Yoon Suk Yeol committed insurrection.

A detailed look at each stage of the impeachment process against President Yoon.

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