North Korea's military fired a missile Wednesday that crossed the disputed maritime border with South Korea for the first time since the countries' division in 1948, officials in Seoul said.

The big picture: The short-range ballistic missile, one of at least 10 that Pyongyang fired, landed some 37 miles from the South Korean city of Sokcho — causing air-raid sirens to sound on Ulleungdo island and Seoul's military to respond by firing three missiles, per the BBC.

Why it matters: The major escalation in Pyongyang's recent spate of missile launches breached the Northern Limit Line, the de facto sea boundary drawn by the UN at the end of the 1950-53 Korean War, landing some 16 miles south of the demarcation line.

• South Korean President Yoon Suk-yeol said Pyongyang's actions were an "effective act of territorial encroachment," Reuters reports.
• North Korean state media said leader Kim Jong-un personally oversaw the previous missile tests that it described as "tactical nuclear" drills representing a "warning" to the U.S. and South Korea — which have this week been holding their largest-ever joint air exercises.

Flashback: The militaries of North and South Korea exchanged warning shots along the maritime border last week.

Between the lines: "North Korea firing missiles in a way that sets off air raid sirens appears intended to threaten South Koreans to pressure their government to change policy," Leif-Eric Easley, a professor at Ewha University in Seoul, told AP.

• "North Korea's expanding military capabilities and tests are worrisome, but offering concessions about alliance cooperation or nuclear recognition would make matters worse."

Editor's note: This article has been updated with new details throughout.