North Korea has reportedly fired a missile from a submarine in the Sea of Japan in response to joint military exercises between the United States and South Korea.

This muscle-flexing was expected, given the public warning given by Kim Yo Jong, the younger sister of North Korean ruler Kim Jong Un, on March 6, as The Dallas Express previously reported.

“We keep our eye on the restless military moves by the U.S. forces and the [South] Korean puppet military and are always on standby to take appropriate, quick, and overwhelming action at any time according to our judgment,” Kim Yo Jong had cautioned, per the state-run Korean Central News Agency.

Units from both the U.S. and South Korea have engaged in one of the largest joint exercises in recent history at Camp Humphreys, located approximately 40 miles south of Seoul. Dubbed Freedom Shield, the drills began on March 13 and will run for 11 days.

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While the U.S. and South Korea have been conducting joint military exercises for decades, they had been scaled down in recent years to aid efforts to negotiate with North Korea. The recent field exercises of Freedom Shield are the largest since 2018, according to CBS News.

Within 24 hours of the commencement of Freedom Shield, North Korea launched a cruise missile from one of its submarines in the vicinity of the North Korean port city of Sinpo.

Given North Korea’s recent record of land-based launches this past year, the underwater firing was unusual.

“At a time when its efforts to build [bigger submarines] have reported little progress due to the sanctions, North Korea wants to show it’s still almost developed the types of missiles that can be fired from a submarine,” said Moon Keun-sik, a submarine expert who teaches at Kyonggi University in South Korea, per NPR.

Although only one missile was confirmed by South Korean military officials, DPRK Today, a Chinese-based state-backed news agency, reported that there were two.

The missile launched from an 8.24 Yongung submarine. The missiles were said to have flown for over two hours and apparently struck targets approximately 930 miles away.

Kim Dong-yub, a professor at the University of North Korean Studies of Seoul, told NBC News that the launch details indicate that Japan, along with the U.S. military bases located in Okinawa, could be targeted by North Korean cruise missiles.

Additionally, he suggested that the missiles could potentially reach the U.S. Pacific territory of Guam if the submarine is capable of operating at a greater distance from North Korean waters.