Abstract: It appears that U.S. and Republic of Korea coordinated closely to deal with the Korean Peninsula issue before and after EC-121 Incident of 1969. In fact, new evidences reveal a profound suspicion between Washington and Seoul. The incident deepened the ROK’s mistrust of U.S. security commitment and led up to new U.S.-ROK confidence crisis. Finally, Seoul sought ways to improve relations with DPRK and achieve self-reliance in national defense. On the other hand, EC-121 Incident obliged President Nixon shelve his plan of reducing U.S. armed forces in ROK. Washington considered the implementation of U.S. presence reduction until November 1969. The main reason for U.S.-ROK confidence crisis lies in the obvious difference of cognition and conduction for Korean peninsula affairs between Washington and Seoul. The case study also indicates that although common enemy may promote the two countries or regions with very different capabilities to form an alliance, they do not necessarily share similar strategic thinking and judgment.