Lee Suyoon, a 20-year-old student, was at home just outside of Seoul on Dec. 3, sharing late-night fried chicken dinner with her mother when her phone started buzzing. A flurry of messages from her friends alerted her that martial law had been declared in South Korea and soldiers were breaking into the National Assembly.

Ms. Lee, who like others her age entered adulthood largely detached from politics, dismissed the notion that there could really be a military dictatorship, and she went to bed.

Everything changed the next day. As her Yonsei University classmates talked nonstop about what had happened, they shared videos of soldiers clashing with lawmakers, and news articles about military generals testifying before Parliament. After seeing that other campuses were galvanizing protesters, she felt compelled to act.

By Saturday, Ms. Lee had persuaded four friends to join her and tens of thousands of others to demand President Yoon Suk Yeol’s removal outside the National Assembly, while a vote on an impeachment motion was underway inside.

“The martial law declaration has forced me to realize that democracy is important,” she said after the protest, and that it is also fragile.