SEOUL, South Korea (AP) — North Korea's rapidly accelerating nuclear weapons program is beginning to pose a grave challenge for liberal South Korean President Moon Jae-in, whose dovish proposals for engagement have been met by silence and two intercontinental ballistic missile tests in less than a month.

Throughout the election campaign and his presidency that began in May, Moon has persistently expressed a desire to reach out to North Korea. But in the wake of the North's latest ICBM test, a stern-looking Moon on Saturday sounded more like his conservative predecessor as he ordered his troops...