SEOUL—In the dead of night, at 3:17 a.m. on Wednesday, a South Korean air force Boeing 737 early warning aircraft detected the first missile launch from North Korea in more than two months.

Six minutes later, the army’s ground-based launchers, navy Aegis destroyers and air force F-16 jets began firing missiles into the waters off eastern Korea, in what was meant as a demonstration of Seoul’s readiness for conflict and its ability to hit back.

The display appeared largely successful, but security analysts said South Korea may not be able to respond as swiftly or accurately in a real wartime scenario.

North Korea launched its latest intercontinental ballistic missile—a new type of device that experts say is capable of hitting Washington—from Pyongsong, about 20 miles north of the capital Pyongyang, a site the regime hadn’t previously used for weapons tests.

According to a detailed account on Thursday from South Korea’s defense ministry, the location in the sea targeted by its military was calibrated to match the distance to the launch site to show it could hit the site if it chose to do so. President Moon Jae-in had already been notified.