Why War Is A Young State’s Game

The world is experiencing an era of unprecedented demographic change. Due to major reductions in fertility levels and significant increases in life expectancies over the course of the last century—and especially since the end of the Second World War—almost all countries are growing older, many at unprecedented rates and extent. In 1950, the median age of the world population, according to the United Nations, was 22. In 2050, it will be 36, and in 2100 it is forecasted to be 42. By 2063, the UN predicts that there will be more people in the world ages 65 and older than under 15, which will be a historical first. Over 40 countries are already depopulating due to advanced stages of aging. By 2050, nearly 90 will be.

Analysts and policymakers frequently decry population aging’s domestic costs, especially likely slowing economic growth and massive new public expenditures for elderly welfare. But aging has a major yet largely unrecognised international benefit: it significantly reduces the likelihood of international war. The twenty-first century, especially as the decades pass, is thus likely to be an increasingly peaceful one.

Aging will significantly reduce states’ military capabilities available for war while also boosting leaders’ and citizens’ preferences for peaceful foreign policies. At the same time, the effects of aging will help prevent the emergence of a power transition between the United States and China, which would be a development that is particularly likely to devolve into armed hostilities. If an aged country does initiate war (as occurred when Russia invaded Ukraine in 2022), the effects of aging will create major barriers to military success.

Aging affects negatively countries’ capabilities in three ways. First, aging slows states’ economic growth, which damages both the quantity and quality of military resources. Aging harms states’ economies by shrinking the number of workers in a society, slowing productivity growth, and reducing domestic consumption. The first two of these variables are the defining ones of gross domestic product (GDP) and GDP per capita. The last is a key engine of economic dynamism. Europe’s working-age population (ages 15 to 64) will shrink by roughly 17 percent between 2020 and 2050; eastern Asia’s working-age population will shrink by 24 percent. Great powers will not be immune from this trend. To the contrary, some of these states will suffer the greatest losses. Between 2020 and 2050, Japan’s working-age population is expected to shrink by 28 percent, China’s by 23 percent, Russia’s by 19 percent, and Germany’s by 17 percent. A key reason why aging hurts both productivity and consumption is rooted in a common cause: the large-scale substitution of less productive and lower-spending individuals (typically people in their 50s and older) for more productive and higher spending ones (typically people in their 40s).

Second, aging creates powerful crowding out dynamics, including for military spending, as governments feel compelled to spend much more money on elderly welfare as the number of seniors in states explodes. Analysts predict that many developed countries’ public spending on pensions, healthcare, and long-term care for the elderly is on track to be roughly 25 percent of GDP by 2050. Quantitative evidence from both democratic and authoritarian regimes indicates that when governments spend roughly one quarter of their budgets on social welfare, including for the elderly, the likelihood that they will initiate a militarised dispute declines significantly as the crowding out of military expenditures becomes pronounced and leaders endeavor to conserve scarcer resources. Many aged countries either are already or soon will be past this threshold.

Third and finally, aging reduces the number of people in prime military-age cohorts. This last development tends to shrink states’ military capabilities by reducing the quantity and/or quality of military personnel, or by creating crowding out forces within military budgets (as governments try to attract and keep recruits by increasing military personnel salaries and benefits) that could decrease the quantity and quality of armaments and equipment. Aging states with shrinking recruitment pools have consistently had trouble meeting their military personnel goals despite frequently relaxing standards and increasing compensation. Japan from 2000 to 2021 was able to fill an average of only 93 percent of its authorised personnel positions. This percentage was even lower for Russia, which consistently failed in the 2010s to reach its recruitment targets for either conscripted or contracted soldiers.

Few states are likely to be harmed by aging more than China, which is aging faster and to a deeper extent than any country in history. Even if China’s fertility rebounds by roughly 20 percent from its current very low level of 1.0, the country will have approximately 138 million fewer citizens in 2050 than it did in 2020 and 788 million fewer by 2100. By 2050, China’s median age will be nearly 52 and close to 31 percent of China’s population will be ages 65 and older. China’s aging crisis will create a myriad of major economic, social, military, and political challenges, including dramatically slowing economic growth and ballooning elderly welfare costs. The fact that China’s aging crisis is significantly worse than similar problems for the United States (which are substantial) also creates a powerful force for the continuation of U.S. relative power dominance. Studies have shown that the probability of international conflict grows when either the dominant country in the system anticipates a power transition in favor of a rising state or states, or when such a transition actually takes place. By adding substantial support to the continuation of U.S. power superiority, population aging works against either outcome transpiring.

Although it is possible that decreasing capabilities due to extensive aging could lead to increased war due to the effects of closing windows of opportunity, history indicates that this will not be the most common result. War in response to declining capabilities is the exception to the rule, not the rule itself. Aging countries are, on the whole, unlikely to be an exception to these tendencies. To the contrary, because population aging reduces states’ preferences for war at the same time it shrinks the capacity to do so, demographically old states are particularly likely to respond to power decline with less bellicose foreign policies.

Aging reduces states’ preferences for war in three ways. First, aging is likely to raise levels of casualty aversion by significantly increasing the costs associated with soldiers’ deaths. When the pool of potential military recruits shrinks as societies age, casualty aversion is likely to intensify as people try to conserve an increasingly scarce resource. Growing military recruitment problems will not automatically result in high casualty sensitivity and consequent high barriers to war, as evidenced by Russian leaders’ disregard for soldiers’ lives in the Ukraine war. Military personnel deficiencies that are in large part rooted in population aging have, though, significantly handicapped Russia’s ability to conquer Ukraine. Russia’s callousness to its soldiers will also intensify the country’s aging crisis, thereby making peaceful foreign policies even more likely in the long run.

Second, aging empowers a demographic group—seniors—that tends to be significantly more pacific in terms of international preferences than other, younger groups. Decades of cross-national survey data confirm that there is a strong relationship between increasing age and a decreased willingness to support the use of force abroad. Seniors’ political interests provide the most plausible explanation for this relationship. The more war prone country’s foreign policies, the less money that will be available for public spending on older individuals. As older-age groups increase as a percentage of states’ populations, their political power will grow, thereby making war less likely over time.

Third, aging increases the probability of domestic discontent and divisions, which are likely to draw leaders’ resources and attention inward and away from war. These divisions are also likely to deny states sufficient political consensus to pay for costly international policies. Population aging will tend to enhance discontent due to economic grievances caused by slowing economic growth and increasing budget austerity. Aging will tend to raise levels of political polarisation because older people tend to be more partisan than younger individuals, and by likely facilitating the demographic decline of a country’s dominant ethnic group in relation to minority ones. The latter outcome will frequently result in growing within-country inter-ethnic tensions.

Population aging will not prevent all wars, as Russia’s invasion of Ukraine demonstrates. Aging, though, will create powerful barriers to aggression as capabilities decline and preferences for peace increase. As aging spreads and intensifies across the world over the course of the coming decades, the barriers to conflict will also become increasingly strong.

This article’s argument is drawn from Mark L. Haas, The Geriatric Peace: Population Aging and the Decline of War (Oxford University Press, 2025) and Mark L. Haas, “The Coming Pax Geriatrica: Aging Societies and Depopulation Will Lead to Fewer Wars,” Foreign Affairs, July 24, 2025.

Disclaimer: Views expressed are of the author(s) and do not necessarily reflect the views of The Statecraft Institute.