“Things have come down to the Triarii." This is an old Roman saying, meaning that the situation has come to its bitter end. When the legions were essentially made of conscripted citizens, the Triarii were the oldest and wealthiest soldiers, and in battle they stood behind the lighter and younger Hastati and Principes. Usually, the Hastati were employed first, followed by the older and wealthier Principes, which usually were enough to win the battle. Having to commit the Triarii—the oldest, most influential citizens—into the mêlée meant that the situation was dire, and that victory was to be attained at all costs.
The trend of relying on the youngest troops to do the bulk of the fighting has generally dominated warfare since the first states warred, but so has the demographic trend of having a predominantly young population with positive growth rates. The latest demographic projections show a general increase in the median age of populations across the entire world and a decrease in their growth rate. This can be attributed to the increasing longevity and the declining fertility of populations, specifically in developed countries. For the first time in history, barring plagues and other black swan events, total population might even begin to decrease, and over one fifth of the world’s population could be above 65 years old in 2050. Aging is relevant, as it is both a cause and an effect of a great number of changes society is experiencing. For instance, populations with negative growth rates can make traditional welfare systems unsustainable. Or when coupled with other recent trends, such as automation and interconnectedness, even the nature of work could be transformed.
We then seem to be living through a paradigm shift, but although we perceive change everywhere, there is also a great deal of continuity. War has been a part of our history since man began to organize in political entities, and while the current global system makes large-scale, interstate wars less likely, other forms of conflict are growing in frequency, fought within states by irregular forces which often serve as proxies for larger entities. As with the other spheres of human activity, warfare is also changing, but war itself remains.
But how does aging change warfare? If the growth rates of developing countries start to decrease, the most evident effect will probably be a reduction in the percentage of fighting-age males worldwide.
If we analyze the effects of this decrease through a reductionist perspective, we could be led to think that it would cause a general decrease in the level of violence worldwide: wars are mostly fought by fighting-age males. A decrease in growth rate worldwide will eventually lead to a decrease in the percentage of military-age individuals, which could in turn make societies less inclined to employ violence, fearful of losing a significant percentage of their youngest adults. Cynically, one can even risk saying that a human life is worth much more today than ever before, which results in a higher reluctance to deploy “boots on the ground” or any military activity which risks human lives. One can already see such reluctance in the deployments and operations of developed countries, which tend to use standoff solutions such as airpower or local proxies.
This almost linear relationship between variables doesn’t consider that aging occurs in a system, an increasingly more interconnected, ever-changing, global system. When we consider other system-wide trends, such as technological evolution, globalization, and the resulting change in patterns of conflict, the linear relationship between aging and war disappears: technological evolution, through increasingly autonomous weapons systems, could cancel out the decrease in violence that we attributed to the aging of populations, or even reverse it.
What aging and those systemic trends—technological evolution and the changing patterns of conflict—all point at is the deep change in who will fight the coming wars. Conflicts will require the regular use of professional, specialized troops like the Triarii, and militaries or other armed entities will have to adapt to a world where Hastati are no longer the less valuable and more abundant troops. Militaries based on conscription could well become a thing of the past, and for some countries there may not be enough individuals to satisfy the manpower requirements. Furthermore, in an increasingly interconnected world, alliance defense becomes more important than solely national defense; this also favors investing in the Triarii.
Thus, when coupled with the systemic trends mentioned above, aging could provide the perfect opportunity for a paradigm shift in military organization and culture. We should strive to build leaner and shallower organizations which completely leverage the capabilities of every available soldier instead of relying on the bottom ranks for cannon fodder (i.e., to create the critical mass of manpower required to fight a peer adversary in an interstate war). Since the required mass will tend to be increasingly provided by autonomous systems, this frees up available personnel to the positions requiring human ingenuity, intuition, and creativity. These less abundant—but also increasingly educated, specialized, and empowered by technology—soldiers blur the line between officer and enlisted, requiring change in the personnel systems and deeply affecting the organizational culture of military forces.
A possible end state could be a force closer to a modern gendarmerie than to a nineteenth-century army, in the sense that each police officer has a much greater degree of independence than an equivalently ranked enlisted soldier in a traditional army. Such forces are usually organized in small, lean, regionally-oriented, highly autonomous, and decentralized modules ideal for dispersed operations amongst the populations, requiring little guidance besides overarching guidelines like “maintain the peace, enforce the law.” They are also complemented by specialized modules for niche tasks that can be added to each unit as required. Organizing future military forces along these lines will only be truly possible once the mass required for the increasingly small likelihood of interstate warfare between peer adversaries is outsourced to autonomous weapons systems, which would allow a higher level of exploitation of the decrease due to aging amount of human potential.
Deep changes to the structure of military forces are not in and of themselves a bad thing: some aspects of the current paradigm in military organization and culture can be traced back to the French Revolution or even earlier, and are now increasingly distant from the contemporary world. As societies change—their demography, their technology and their conflicts—so must their armed forces. We should then embrace this change, and use the aging of the population as a catalyst in the transformation of those who defend it.
Artur Varanda is a Portuguese Army officer especially interested in the future of warfare. The opinions expressed in this essay are solely his own.
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 I periodically encounter this Roman saying when reading about Rome’s armies before Gaius Marius’ sweeping reforms (2nd Century BC). While I cannot cite a definitive, primary source for it, it is not hard to imagine how this witticism originated after reading about the organization and tactics of the pre-Marian Legions. I found The Roman Army: The Greatest Fighting Machine of the Ancient World (Osprey Publishing, 2010) to be a good, well-researched book on the subject. Page 72 in particular offers a good look on the role of the Triarii.
 Every assumption about demographic trends was based on the UN report World Population Prospects, the 2015 revision ( https://esa.un.org/unpd/wpp/ ). I avoided citing precise numbers for 2050 as a measure of caution: since the data-collection processes, assumptions and computing methods used by the UN researchers are mostly opaque to the layman, predictions thus made must be taken with a grain of salt. So, while we can’t directly discuss the precision of the predictions, we can identify trends, which are still helpful in illustrating future challenges.
 These claims are supported by research conducted by the Oslo Peace Research Institute. One graph in particular illustrates how intrastate conflict dominates the system nowadays: https://www.prio.org/Projects/Extensions/ConflictTrends/Graphs/?xitem=1631&handler=Project#List3Tab1.
 This is a tricky concept, built on my assumptions on the subject; naturally, it lends itself to questioning. A cursory look at the history of warfare show us that conflicts were mainly fought by males in the 18-50 age range (i.e., males able to sustain the level of physical effort required for combat). This is why defining a precise age range is tricky, and frankly, unnecessary to advance the argument that the percentage of fighting-age males is decreasing.
 Mike Benitez and Mike Pietrucha’s Political Airpower series at War on the Rocks discusses this problem in detail: https://warontherocks.com/2017/01/political-airpower-part-iii-boots-off-the-ground/ .
 Which, as frightening as it may sound, is a development that will probably happen whether we like it or not.
 A Gendarmerie is a police force comprised of military personnel. France’s Gendarmerie Nationale and Portugal’s Guarda Nacional Republicana are examples of gendarmeries.
 This will sound familiar to the proponents of auftragstaktik.
 The assumption that near-peer, interstate wars will become less likely or even disappear altogether is very dangerous. The occurrence of such a conflict when no one will be expecting it fits the definition of Black Swan as advanced by the philosopher Nassim Nicholas Taleb: “Black Swans are large-scale unpredictable and irregular events of massive consequence” (Nassim Nicholas Taleb, Antifragile, 2012, p. 6). Even so, it is worthy to remember Brigadier Simpkin’s concept of “unusability” of conventional forces (Race to the Swift, 1985): that interstate, “conventional” wars have the potential to become so costly, so destructive, and to escalate so quickly to a nuclear exchange that regular (as opposed to “irregular”) forces become unusable—their role is to deter possible attempts of achieving a fait accompli solely by their existence and credibility. Thus, it is not too far-fetched to think that such deterrence can be accomplished with an increasingly autonomous cohort of systems (in many ways, it already is—think about the ever-increasing ecosystem of drones and missiles), releasing the humans for concrete issues and operations