Purpose: This article examines the potential impacts on the security of the Korean Peninsula(KORPEN) due to the slumping defense workforce in the depopulation and aging era and suggests the impact of the Population Dead Cross Phenomenon(PDCP) to be a considerable factor for the future direction of the ROK-U.S. combined defense system. Method: As a research method, the framework of analysis is to examine the impact of the PDCP and the DPRK military threats on the security of the KORPEN then recommend the future direction for improving the alliance relations in the era of Operational Control Transition(OPCON-T). Due to the limitations and characteristics of this research, this study focused on literature research, official documents from the Republic of Korea(ROK) govern-ment and the Ministry of National Defense(MND), academic and dissertations, forums, and seminars related to the demographic and national security studies. Results: The Republic of Korea is the most rapidly advancing country globally, with a low-birth rate and hyper-aging society. The PDCP in the ROK society is a serious security concern to its conscription-based system. Onlook-ers expect the low fertility rate to shake the national defense policy of the conscription system by causing a decrease in the working-age population and a mission-critical decrease in the available national defense work-force. The continued reduction of the ROK defense manpower may raise questions about how to effectively com-mand the future combined defense forces after OPCON-T. Conclusion: In a broader sense, the decline of the ROK population due to the PDCP is a social problem that desperately needs an essential solution. This social problem is also a reflection point for the ROK and the U.S. defense and national authorities, developing the future direction for the ROK-U.S. combined defense system. If the chronic diseases of the PDCP are ignored, it will directly threaten the security of the KORPEN. The ROK and the U.S. should consider multifaceted factors from various angles and prepare for binational decision space to prepare for the realpolitik in shaping the future landscape of the ironclad ROK-U.S. military alliance.