Just six months ago, South Korean President Moon Jae-in was stuck in a downward spiral. His scandal-plagued justice minister had resigned after only five weeks on the job, prompting Moon to issue a public apology. The economy was sputtering, relations with neighboring Japan were at rock bottom, and Moon’s signature policy of détente with North Korea was going nowhere. In October, his approval ratings sank to a historically low 39 percent.

It took nothing less than an unprecedented public health crisis to reverse Moon’s fortunes. Widespread approval of his administration’s response to the coronavirus pandemic led South Korean voters to reward Moon with a historic landslide victory in legislative elections last week. His left-leaning Democratic Party, together with a smaller affiliate, now controls 180 of the 300 seats in the unicameral National Assembly—an unprecedented majority in the history of South Korean democracy. ...