What changes has the Cheonan incident brought about in the Northeast Asian region? How and to what extent have those changes shifted as time goes by? I find that the Cheonan sinking has played a key role in deepening the ideological chasm in South Korea between conservatives and progressives. It has also become a serious obstacle to the improvement of South Korea-North Korea relations, and has been the catalyst for the emergence of a Cold War-like rivalry between the US-South Korea-Japan and the China-North Korea-Russia blocs. However, relations among neigh-boring countries in Northeast Asia have shifted over time, including significant improvement in China-Republic of Korea relations and worsening of relations between China and North Korea and South Korea and Japan. KEYWORDS: Cheonan incident, ideological divide, South Korean politics, inter-Korean relations, Northeast Asian security relations.ON MARCH 26, 2010, THE REPUBLIC OF KOREA (ROK OR SOUTH Korea) navy corvette PCC-722 Cheonan sank while conducting a patrol mission in the Yellow Sea just south of the Northern Limit Line near Baekryeong Island. As a result, forty-six out of 104 crew members were killed while fifty-eight were rescued. Following an investigation, a Joint Civilian-Military Investigation Group (JIG) consisting of experts from the United States, the United Kingdom, Australia, Sweden, and South Korea, announced on May 20 that the Cheonan had been sunk by a North Korean torpedo attack. What changes has the Cheonan incident brought about in the Northeast Asia region? How and to what extent have those changes shifted over time?The JIG's official report, instead of leading to a consensus view among South Koreans, sparked a debate over the cause of the incident. The already deep ideological chasm between con- servatives and progressives in South Korean society worsened. The sinking of the Cheonan also further undermined interKorean relations and served as the catalyst for the emergence of a Cold War-like rivalry between the US-ROK-Japan and the China-North Korea-Russia blocs. Along with these changes in regional security dynamics, the tragic incident also exerted significant influence on interstate relations among neighboring countries in Northeast Asia. In the short run, the Cheonan sinking weakened China-ROK, US-DPRK (Democratic People's Republic of Korea, or North Korea), and US-China relations, whereas it helped strengthen China-DPRK, US-ROK, US-Japan, and ROK-Japan relations. Further shifts have occurred since the incident as China-ROK relations have significantly improved while China-DPRK and ROK-Japan relations have notably worsened.In this article, I first examine the widening ideological gap between conservatives and progressives in South Korea after the sinking of the navy ship by exploring the hotly contested points of the JIG's report. I then look into the impact of the Cheonan incident on Northeast Asian security relations and how and to what extent the changes produced by the incident have shifted over time.The Ideological Divide in South KoreaA Catalyst for Deepening MistrustIdeological disputes between conservatives and progressives in South Korea are hardly new. Since the end of the Korean War, the two camps have mainly clashed over policies toward the DPRK and the security role of the United States. Conservatives give top priority to fostering a close partnership with the United States and maintaining vigilance against the threat from North Korea. Progressives, however, radically oppose that narrative, seeing the North more as a kin nation with which to be reconciled and the United States as a disruptive interloper (Chae and Kim 2008, 77).This ideological divide became more evident after progressive President Kim Dae-jung (1998-2002) launched an engagement policy toward North Korea, called the Sunshine Policy. Conservative politicians, intellectuals, and media strongly opposed the Kim government's initiatives, criticizing him for not demanding reciprocal actions by the DPRK. …