The 2010 sinking of the Cheonan and other provocations by North Korea in the Yellow Sea have never had a proper response from the ROK. For example after the 2002 West Sea Naval Battle that saw six ROK sailors dead the North Koreans celebrated the aftermath while the ROK did nothing in response. In fact the ROK President did not attend the memorial ceremony and surviving family members were treated poorly in an effort to downplay the provocation. With the new ROK defense chief it looks like any future provocations will have a serious response:

South Korea’s defense chief on Tuesday instructed Navy officials to mercilessly bury North Korean sailors at sea in the event of another North Korean provocation.
Defense Minister Shin Won-sik made the remark as he inspected the new 2,800-ton ROKS Cheonan frigate, which was deployed for operations to the headquarters of the Navy’s Second Fleet in Pyeongtaek, 60 kilometers south of Seoul, on Saturday.
Shin told Navy officials and sailors to “mercilessly bury (the enemy) at sea if the enemy stages yet another provocation” after he paid tribute to 46 fallen sailors at the memorial monument at the headquarters of the Navy’s Second Fleet.
In 2010, North Korea torpedoed the 1,200-ton-class Cheonan corvette near the western Northern Limit Line, the de facto inter-Korean sea border, killing 46 South Korean sailors. A Seoul-led multinational investigation concluded that Pyongyang torpedoed the Cheonan warship, but the North has denied its involvement in the incident.
Yonhap
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