South Korea’s declining fertility rate is raising concerns for the country’s defense—and has important implications for the future of the US-South Korea defense relationship. In June, President Yoon Suk Yeol called South Korea’s rapidly aging society a “national emergency” after another record-breaking low fertility rate was announced. The nation has the lowest fertility rate of all OECD countries and one of the lowest in the world, at 0.72 children per woman. Countries need a fertility rate of at least 2.1 children per woman—in the absence of immigration—to maintain a stable population.
All South Korean men between the ages of 18 and 35 are required to serve 18–24 months in the military, but the number of 20-year-old men conscripted is expected to fall from 226,000 in 2025 to roughly 130,000 in 2040, according to the Republic of Korea’s 2022 Defense White Paper. Sangmyung University Professor Chou Byung-ook calculates that the South Korean military needs to onboard approximately 200,000 soldiers a year to sustain current troop levels. The demographic crisis has propelled South Korea’s Ministry of National Defense to look for alternative solutions, by increasing compulsory service recruitments and using modern weaponry in lieu of manpower.
Expanding Options for Military Personnel
The Ministry of National Defense has introduced proposals that would expand recruitment eligibility for lower-level service positions. One included mandating that women complete military service; South Korean women are not currently, and have never been, require to serve. Seoul has also prioritized expanding its reserve system, hoping to create a larger, more experienced force. The 2022 Defense White Paper reported that the number of short-term reservists increased from 79 in 2014 to 3,804 in December 2022. However, this number does not look promising for future manpower, given projected shortfalls. It is possible that reservists could maintain relations with their US counterparts, potentially strengthening the bilateral military relationship. The Ministry of National Defense is continuing to evolve its system to adopt new personnel and maintain technologically advanced reservists to counter a declining number of compulsory recruits.
Modernizing Military Technology
The ministry also looks to technology to partially replace manpower. South Korea is modernizing its defense system under what it calls the “slimmer yet smarter” 2022 Defense Innovation 4.0 plan, a high-tech effort to circumvent the impacts of the demographic decline. This five-year plan focuses on labor-saving efforts such as artificial intelligence, autonomous vehicles, robotics, and manned-unmanned teaming. An article in the Korean Journal of Defense Analyses stated that the Defense Innovation 4.0 structure emphasizes “open innovation” through military-to-commercial collaboration. The mixture of high technology and cross-collaboration between defense and industry could create opportunities for an increased US-South Korea military partnership. Also, after the Ministry of National Defense decreased its number of standing troops, South Korea has focused on improving its operational weaponry. In June 2024, South Korea ordered 20 KF-21 fighter jets, upgrading its older models as it turns to technology equipped for manned-unmanned teaming.
Future-Forward Planning for the US-South Korea Alliance
US-South Korean security cooperation is critical to both countries. Their militaries collaborate on several bi-, tri-, and multi-lateral exercises that focus on deterrence, logistics, and interoperability. Freedom Edge, Freedom Shield, and Ulchi Freedom Shield are critical exercises that the two have conducted to enhance their mutual response and defense posture towards North Korea. The Republic of Korea’s Ministry of National Defense and the US Department of Defense have committed to a deeper combined defense posture through ministerial meetings, policies and dialogues, such as the Korea-U.S. Integrated Defense Dialogue held in April.
If the effects of the demographic crisis on the South Korean military do not ebb, Seoul may seek strength by continuing to build deeper defense relations with its allies and partners. After a Trilateral Ministerial Meeting in Singapore several weeks ago, the United States, South Korea, and Japan “pledged to develop a trilateral security cooperation framework within the year” to institutionalize senior-level discussions, exchanges, and information-sharing regarding Northeast Asian threats. Additionally, in July, the US Air Force temporarily assigned 31 F-16 fighter jets in South Korea to test training effectiveness and combat capability of manpower, maintenance, and logistics in a failed deterrence scenario. South Korea hopes that such support from allies will weaken the demographic decline’s effect on defense.
As the US military plans its future defense objectives, there are several questions it should consider with regards to South Korea and its demographic crisis:
- How does South Korea plan to supplement its military, beyond the plans created in 2022?
- What are some implications of the Defense Innovation 4.0 plan’s “open innovation”? Is it likely to foster deeper US-South Korea defense relations?
- What effect will South Korean reservists have in building South Korea’s manpower? What are some ways reserve forces could positively affect the US-South Korea relationship?
- What effect might declining South Korean troop levels have on United States Forces Korea?
- What objectives may change as South Korea’s military numbers fall? Will the US role in these objectives be affected, and in what capacity would it change the relationship with South Korea?
- How might South Korea’s declining manpower affect its attitude toward acquiring nuclear weapons?
Washington should keep close watch on the progress of the US-South Korea relationship as the demographic crisis and its related challenges threaten to affect security on the Korean Peninsula.
