There were traits of this type of demographically preventive war in Israel’s attack on Egypt in 1956. Although the architects of war in Tel Aviv, notably Ben-Gurion and Dayan, wanted to strike President Abdul Nasser’s regime before it received and absorbed a huge quantity of Eastern European arms, they were also mindful of Cairo’s growing numbers. This was not just because Egypt’s own population was growing more quickly than the Jewish state’s—Egypt has long had one of the highest birth rates in the world, standing at around 2.5 to 3 percent annually—but also because Nasser seemed poised to win the allegiance of the Arab masses outside Egypt’s own borders: with his immense personal charisma and his willingness to embrace the Palestinian cause, the Egyptian leader seemed ready to realize the pan-Arab vision that he had outlined in his writings and speeches.

Depopulation Wars might also be fought not just by numerically depreciating states but also against them. This may simply be because, as Price and Prévost-Paradol feared, a state with a falling population is less able to field an army of the quantity and quality it needs to defend itself. One historical example is the fate of Ancient Rome: it seems likely that its population began to fall from its peak in the time of Augustus, rendering it increasingly more vulnerable to foreign attack at the hands of enemies such as the Gauls and the Visigoths.

In the contemporary world, even the most technologically sophisticated armed forces still require a certain level of manpower whose shortfall renders them vulnerable to a larger, if less advanced, enemy. This is a view held by, for example, British military planners, who in 2015 admitted that falling levels of military recruitment rendered the UK “dangerously vulnerable to external aggression,” emboldening an enemy to initiate hostilities or escalate an existing conflict. Demographics help to account for this shortfall of numbers. In 2005, as the British and their allies faced increasingly belligerent enemies in Afghanistan and Iraq, another government report noted that the armed forces “are not meeting their recruitment targets consistently [because]…an ageing population will mean a decline in numbers of individuals of working age over the long term.” However, this needs to be put in context: the report also noted that there were a host of other factors that caused or accentuated this crisis, including a buoyant economy that provided alternative careers and higher rates of obesity. In addition, less important than declining demographics is a changing population: the report noted a steady or falling birth rate in traditional areas of recruitment but rising birth rates amongst ethnic groups “which historically [have] not been attracted to a career in the Armed Services.”

THERE IS, instead, a more important reason why any state with a diminishing or even stable population is vulnerable to external aggression: Its armed forces may lack not the manpower but rather the necessary financial resources. This is not just because of a falling GDP but more specifically because it has more pressing priorities than defense expenditure. Above all, if a state’s population is ageing while its birth rates are stable or falling, thereby lowering tax revenues, then health care and social security budgets become an urgent priority that demotes other, less pressing demands.

Taiwan provides an example of how demographic decline is apt to create a demand for this type of “butter” over “guns.” Its defense expenditure fell steadily from at least 2008 until 2017, stabilizing at around 2 percent of GDP. But this prioritization was and remains closely linked to its numerical decline. The Taiwanese birth rate has not reached replacement levels since the early 1980s and today, standing at 1.17 percent, is one of the lowest in the world. In August 2016 a governmental report predicted that the island’s population will start to decrease within five to ten years.

This of course has far-reaching implications. At the moment there are 5.6 people of working age for every elderly person; by 2061 this number will have plummeted to 1.3. This means that the Taiwanese government is already spending considerable sums of money to provide health and care services for its elderly citizens while offering generous financial incentives, including tax breaks and subsidies, to young couples to have children. Defense expenditure, by contrast, is viewed by many Taiwanese politicians as an expensive luxury. Demographic decline in Taiwan, in other words, could conceivably tempt China to initiate its own Depopulation War against a state that it has claimed, since its formation in 1949, as its own territory.

This reprioritization of expenditure is also affecting countries in the Western world, including the UK and Germany, that are also confronted by a crisis of health and social care. Between 1988 and 2013, UK defense expenditure almost halved as a proportion of GDP. Welfare spending is now nearly six times higher than defense spending, whereas in the mid-1980s, health and education expenditure were at similar levels to defense spending.

Such financial pressures not only render a diminishing state more vulnerable to attack but also make it less inclined to intervene on behalf of its allies and other third parties. Although such a state is likely to maintain a capability to defend its own self, it is less able and willing to sacrifice its own resources for another country, which then becomes isolated and vulnerable to foreign aggression.

An example of this type of Depopulation War was the Arab-Jewish conflict in Mandated Palestine prior to the formation of the state of Israel in 1948. After sustaining such huge losses in World War II—which cost the lives of 382,000 servicemen—the severely over-stretched British Army was scarcely in a position to fight any further conflicts unless they were strictly “necessary.” Equally, the general British public and its government were just as hostile to any such unnecessary commitments: “Someone else should have their turn now [of] this very difficult place,” Winston Churchill observed about Palestine in 1945. Aware that the mere possibility of a British retreat would create a political and military void, both Arabs and Jews escalated their feuding against each other, as well as against the British army of occupation, in a desperate bid to fill it.

In a slightly different way, the Abyssinia Crisis of 1935 can be classified in these terms. As he calculated his chances of capturing the East African state, the Italian dictator Benito Mussolini judged that the population of Great Britain was too elderly to want to make a stand: with twelve million citizens who were aged at least fifty and therefore, in his view, “over the age limit for bellicosity,” he decided that the British nation was agreeably disposed to passivity. However, such a scenario is more likely to unfold because of a lack of human or material resources rather than because of any difference of attitude, real or imaginary, on the part of a relatively elderly population that may have little or no influence on political decisionmaking.

THERE IS also another, more indirect, connection between a falling population and strategic vulnerability. Sometimes a fall in the birth rate can be symptomatic of an affluence that may not be compatible with military values. Since 1945, for example, Germany has had a low birth rate which is thought to reflect its postwar material prosperity and high standards of living. At the same time, its armed forces have suffered from regular recruitment shortfalls: in 2012, one year after conscription ended, the Bundeswehr suffered a severe (33 percent) manpower shortage. Some analysts have called the republic a “postmilitary society” that has a disdain for the discomfort and sacrifices required by military life. This is very arguably borne of an economic prosperity that also accounts for a diminution of the birth rate.

In the years ahead, strategic realignments might prevent some Depopulation Wars from breaking out. For example, some states that are experiencing depopulation, or are at risk of doing so, will be able to strike closer defensive alliances with more affluent states, like China, India and the United States, that are in a position to guarantee their security or else provide economic subsidies that allow them to increase their own defense spending.

However, it is equally possible that serious political rifts could erupt, within and between states, should a depopulating country fail to increase its defense expenditure or refuse to strike such a deal with other states that regard its security as essential to their own.

An example is the relationship between Taipei and Washington, which is committed, under the Taiwan Relations Act of 1979, to defending the island’s sovereignty and security. In December 2016 Deputy Assistant Secretary of Defense Abraham Denmark publicly criticized President Tsai Ing-wen’s defense budget, claiming that “[it] has not kept pace with the threat developments and should be increased.” Such financial pressures may prompt allies, even long-standing ones, to question the value of such alliances and withdraw from them altogether. For example, transatlantic tensions erupted in the summer of 2018, when Secretary of Defense James Mattis suggested that the UK’s low defense expenditure is imperiling the “special relationship.”