The concept of security dilemma is of importance in understanding international relations (IR): It explains why one state's pursuit of security through arms build-ups may trigger others to do the same, because none of them is certain about the other's intentions. When uncertainties accumulate, war seems to be more likely, and even for the state that tries to pursue security in the first place may feel less secure. Current tensions in the Korean Peninsula put parties involved in a security dilemma, wherein, with a lack of mutual trust, their sense of insecurity increases. This paper aims to explore how the security dilemma affects the situation in the Korean Peninsula after the Cheonan Incident in 2010. The security dilemma, as advised in IR literature, cannot be resolved but at most managed and ameliorated. North Korea's nuclearization and South Korea's ambition for unification are a case in point. This is the reason why President Park Geun-hye's call for trust-building between North and South Korea, dubbed Trustpolitik, is worth noting. From an IR theoretical perspective, international institutions and strategic reassurance, more or less, can serve to lessen the security dilemma. However, power politics still trumps the willingness of cooperation among nations.