But given the financial, political and (if it incurs sanctions) economic costs of war, it is unlikely that any country will initiate conflict purely to bolster its prestige. It is instead more likely to undertake risky, even provocative actions that make a clear statement to the outside world. One example is the way in which Russian warplanes “buzz” nato airspace by flying dangerously close. The number of such incidents have spiked in recent years and on one occasion, in April 2016, a Russian jet flew within thirty feet, at an altitude of one hundred feet, of the USS Donald Cook just as the American destroyer was practicing helicopter landings. But such actions can be seriously destabilizing, not least because they risk provoking an “escalation spiral,” or because of the risk of an accident.

Another possible outcome is a heightened military presence and activity in regions that have strategic importance to the outside world: for instance, in recent years the Kremlin has been building more military bases in its Arctic territories, close to disputed areas that it claims as its own sovereign soil. Such actions may be perfectly legitimate under international law but nonetheless send a clear message of belligerence and aggression that aggravate tensions with other countries.

IT IS probably more likely that ideological, one-party states will act in such a compensatory way. If the population of such a state falls, then it is difficult for government leaders to proclaim it is a “paradise on earth.” This, for example, is a claim that the Communist Party propaganda of the Pyongyang regime has long advanced. Such a “paradise” should in theory be conducive of a state of happiness and pride that engenders high birth rates, delivers a material well-being that reduces death rates and attracts immigrants rather than creating emigrants. This was one reason why, in 1937, a furious Joseph Stalin indicted the director of the Soviet Union’s official census with plotting to “make the population low.” And in North Korea in 1984, a state-run publication proudly proclaimed a low death rate: “Our nation,” it exulted, “surpassed the so-called developed capitalist nations… [which is] entirely the glorious result of the great leader, Comrade Kim Il-Sung.” It follows that any such ideological regime may try to compensate for its numerical decline in other ways.

However, a democratic state can also act in a similarly self-aggrandizing way in a bid to compensate for its diminishing birth rate. France, for example, developed a nuclear bomb not just for military and political reasons—to provide its own deterrent, independent of the United States—but to compensate for the loss of its colonies and the consequent fall in demographic power. Prior to the independence of its colonies, which began in the 1950s, Gen. Charles de Gaulle had argued that only an imperial France of one hundred million people could retain its rang (rank amongst nations). But as France’s imperial role waned, successive French governments pursued a nuclear weapons program to compensate: in 1960, the French successfully test-fired a nuclear device, prompting de Gaulle to proclaim his country “stronger and prouder” as a result.

There is another, more important, demographic reason why any country might pursue the nuclear option, thereby provoking regional arms races as well as preemptive military strikes: Any country with a diminishing population is less able, in the long term, to defend itself using conventional means. Most obviously, this is because such a state lacks the manpower to field an army of comparable size to a putative aggressor. Another reason is that it may no longer have the economic and financial weight to sustain a protracted war effort. A logical response to the challenges of a falling population is therefore to develop a nonconventional deterrent: the nuclear bomb. It also makes sense to do this without delay, while the population is sufficiently large and the economy strong enough to sustain the very high upfront costs of doing so.

North Korea’s demographic decline or, at the very least, the regime’s fears of such a diminution, remains central to understanding why it has sought to retain and develop “the ultimate deterrent.” By crossing the nuclear threshold, Pyongyang guards itself against the population falls that are likely to occur in the years ahead. Although there are few reliable statistics about the demographic reality of such an opaque state, estimates strongly suggest a long-term decline. Its population grew significantly in the 1960s before falling sharply in the 1970s and then leveling out: It is currently growing annually at around 0.5 percent. However, even this meager increase is superficial because it is made up more by people living longer than by new births: after peaking in the 1960s, fertility levels have gradually declined and currently fall below the replacement level of 2.1 children per woman.

There are other indications of North Korea’s numerical stabilization and long-term decline. In the sixty years between 1950 and 2010, its share of the entire peninsula’s population has fallen from 35 to 33.6 percent. Furthermore, independent studies have predicted that its overall population, ageing or otherwise, will experience a net decline over the next twenty to thirty years: in October 2016, the Korean Educational Development Institute claimed that North Korea’s school-age population, currently standing at 4.14 million, will decline by 90,000 by 2040. Such decline would render the North Korean regime highly vulnerable to sudden, devastating outbreaks of disease and famine, comparable to the mass starvation that decimated its population in the early 1990s. In a bid to bolster the falling birth rate and make the republic into the “strong and prosperous nation” that his propaganda boasts of, Kim Jong-un has reportedly banned abortion and birth control. At the same time, the development of a nuclear weapons program provides Pyongyang with a degree of security (not least as a bargaining chip) that would guard against long-term numerical decline. It is no coincidence that this nuclear program appears to have accelerated in the late 1990s, when the demographic impact of famine would have become fully apparent.

DEMOGRAPHIC CONSIDERATIONS also impelled Israel’s nuclear weapons program, which was instigated and developed in the 1950s on the premise that the population of the Jewish state would always be outnumbered by its Arab neighbors. At a conference held in Tel Aviv on May 5 1955, Prime Minister David Ben-Gurion argued for the development of the bomb on the grounds that “we must have superiority in weapons because we will never achieve superiority in manpower.” Others held a similarly pessimistic viewpoint and argued that regional demography would always work against Israel: in the course of crucial debates in 1961–2, Shimon Peres and Gen. Moshe Dayan both took this view, urging the rest of Ben-Gurion’s cabinet to pursue a nuclear option rather than rely solely on conventional arms. “Quality versus quantity” was the slogan of this Israeli school of strategic thought.

There are also circumstances in which demographic changes within states can incentivize a government to develop a nuclear arms program. South Africa’s white minority government, for example, initiated its program in or around 1974, at a time when the republic’s white population was—officially—stabilizing at around four million while the black population was dramatically increasing: according to official government figures that generally underestimated the black population, South African whites represented 19 percent of the overall population in 1969 but five years later this had fallen to 16 percent. In addition, the exodus of the white population also looked likely to increase as international condemnation of the apartheid regime and the risk of economic sanctions grew. In such a demographic scenario, the iniquity of white minority rule would become all the more apparent, rendering it even more indefensible before growing excoriation from overseas. However, a nuclear capability gave Pretoria a weapon that could not only fend off external aggressors but also act as a political bargaining chip to wield against the foreign critics of apartheid, most notably the United States: for example, the politically embattled regime in South Africa could have threatened to hand over its nuclear materials to an unstable successor, perhaps with pro-Soviet sympathies.

Instead of pursuing a nuclear option, a demographically diminishing country may instead invest in conventional arms whose “quality” gives it an edge over the “quality” of its rivals and adversaries: As Ben-Gurion had added in 1955, “All those things to do with science, we must do them.”

But this is also destabilizing for the obvious reason that it is likely to create an arms race. Again, Russia illustrates this danger. In January 2018, the head of the British Army, Gen. Sir Nick Carter, warned against the growing technological prowess of the Kremlin’s armed forces, claiming that the quality of their long-range missiles, artillery and capacity to wage electronic and cyber warfare had surpassed those of Western armies. “They know that demography is not on their side,” as Carter told a British defense forum, “so they are developing capability that needs fewer men—for example missiles, drones and two-man tanks.”

A DEPRECIATING state might also undertake another type of Depopulation War. This occurs when it launches a preventive assault on its adversary, calculating that the moment of attack represents its “last chance” to neutralize an enemy that will otherwise become too numerically strong to defeat. A historical example is the German attack on Russia in World War I. In 1914 German planners intended to inflict a decisive defeat on their eastern neighbor before it became not just too economically strong but also too demographically prosperous: between 1900 and 1914, Russia’s population had already grown by forty million and was expected to soon reach 200 million, while Germany’s stood at a relatively meager sixty-five million. “Russia grows and grows. She lies on us like a nightmare,” wrote Theobald von Bethmann-Hollweg, the German chancellor, shortly before hostilities began in August 1914.