What happened?
South Korean President Moon Jae-in gave a stern warning to North Korea stating that it will retaliate strongly against any threat to its people. He urged North Korea to maintain peace in the Korean peninsula and warned against any armed threat. 

The remarks, unusual for a President who has dedicated the last three years in improving relations with the North, came during a live speech, commemorating the 70th anniversary of the start of the Korean War.

North Korea announced earlier this week that Kim Jong-un has suspended military "action plans" against South Korea, while it continues to strengthen its nuclear capabilities. It accused Washington of increasing the conflict between both the countries and rejected any future talks on nuclear arms control.

What is the background?
First, the failure of inter-Korean rapprochement. Moon Jae-in was successful in re-initiating the process of rapprochement in 2018. Failure of US-DPRK summits on denuclearisation of North Korea and the further imposition of economic sanctions on it, lead to a tiff in the Korean peninsula. The rapprochement collapsed after North Korea blew up the inter- Korean liaison office in the border town of Kaesong and threatened military action against the South.

Second, the anti-Kim campaign by North Korean defectors and South Korean activists. The North Korean defectors have constantly been sending anti- Pyongyang leaflets across the heavily fortified border. Though the ulterior motive is to make North Koreans aware of the regime, the anti- Kim campaign has aggravated the tensions between North and South Korea. 

Third, Pyongyang aggressive threats against South Korea. North Korea retaliated with several stern threats against the anti-Kim leaflets. North Korea cut off all communication lines with South Korea and threatened to nullify the 2018 agreements. It also threatened to send troops to the demilitarized zone after rejecting South Korea's offer to send envoys regarding the dramatic destruction of the joint liaison office. 

What does it mean? 
First, the suspension of military "action plan" against South Korea came as a surprise, while the tensions in the Korean peninsula was at its peak. The decisions taken by North Korea seems to be erratic, creating an uncertain situation in the region. Accordingly, North Korea might continue with the same relation with the South in order to maintain consistent pressure. 

Second, President Moon Jae-in made a bold move, calling North Korea to formally end the Korean war in peace overtures. It can be seen as a move to stabilize his approval ratings which hit an all-time low after North Korea blew up the liaison office. North Korea's threat also hit his cabinet, leading to the resignation of the Unification minister. The President's speech can be seen as an attempt to bolster the confidence among his cabinet and the public.