Nature abhors a vacuum; so too does geopolitics. The nation state has conquered the world and it cannot abide an empty place on the map for long.

The collapse of the Syrian state has left behind such a vacuum. The question has never been if it will be filled; short of human extinction, that is inevitable. The question is by who.

Now Turkey has finally invaded Syria: not to smash and grab but to stay, in one form or another.

What’s going on here? Isn’t this just another sad mistake in the long series of sad mistakes in Syria?

Not necessarily. Here’s why.

First, we need to understand power and influence in the twenty-first century

Once upon a time, British gunships could bombard a sultan for less than an hour and overthrow him. Once great armies could mass on borders and threaten havoc and change governments. Geopolitics used to be more brutal but also simpler: those who could amass the most soldiers with the most guns and ships often could change the map as they liked.

But mid-point in the twentieth century, two critical factors shifted power away from troop and ship counts: the advent of nuclear weapons and the population boom.

Nukes ended war as an instrument of power between the great powers. While once a coalition of France and Britain could invade Russia, as they did during the Crimean War, today any attempt to restore the balance of power in the Black Sea would almost inevitably result in Moscow firing nukes and its enemies firing nukes back.

Nuclear weapons made it so that great powers had to fear one another in a way they never had before: Any misread of the situation would result in not merely World War III but the annihilation of the human species. Elites in every nuclear-armed country have absorbed this lesson fully; so too have their citizens. American citizens and Russian citizens may mistrust, even despise one another in some cases, but there are few who wish to actually do anything about it.

But this might have simply meant that great powers could not directly attack one another, not that they couldn’t, as players of the Civilization are wont to do, conquer neutral and weak nation states to grab up more population, territory and resources. This used to be the way nation states expanded: America conquered much of Mexico, Europe conquered nearly all of Africa, Japan conquered Korea, etc., all in the pursuit of power.

But the population boom has put an end to all that. From 1950 onward, the planet’s population has skyrocketed. This has had a few knock-on geopolitical effects: first, it means that elites have more people than ever to manage in their own countries; second, it means that elites who seek to conquer foreign lands have way more potential enemies they must rule over.

The first increases the cost of running the state: more cops, more laws, better-funded and -run institutions, all siphoned power away from military adventures and created forces within nation states that had little interest in building empires. A king in the nineteenth century often had only to contend with his aristocracy and perhaps some budding parliament. Those systems were usually enough to keep Industrial Age populations in line. But today’s populations dwarf the past’s: Britain has doubled from 38 million to 64 million people, for example.

This has also made subduing foreign populations all the more difficult. Defeating an enemy army is a relatively simple affair: push the enemy back from their positions, capture their capital, force their government to surrender or flee. Even today, most militaries are not even close to their World War II size: the United States had some 12.2 million troops in 1945, but only has barely 1.3 million in 2016. Yet it can readily roll into most of the world’s capitals in weeks.

But when it comes to controlling a population after the war has been won, the math is just as tough today as it was in 1945. According to the United States Army, the amount of soldiers needed to pacify a population can range from as little as 2.2 soldiers per 1,000 to as many as 150.

When a population is motivated to fight an occupier, as the Iraqis were, that number slides up beyond the capabilities of today’s footprint-light armies. When a population is not motivated to resist, it slides down: Crimeans overwhelmingly welcomed the Russian occupation and annexation, leaving little room for a resistance movement.

So how does all this affect Erdoğan’s Turkey? First, let’s start with the imperial hangover of the Ottoman Empire

Turkey has spent nearly the whole twentieth century trying to recover from the corrosive legacy of the Ottoman Empire. One of its biggest hangovers is the Kurdish problem. When the Ottomans were empire building, they had neither the need nor the means to Turkify the whole realm. Defeat in World War I shed Turkey of its more populous foreign provinces, leaving a rump Turk-dominated realm in Anatolia. Further ethnic cleansing after the Turkish War of Independence cleared Anatolia of its ancient Greek community while the Armenian genocide dealt with a major potential separatist movement in the east.

But nothing was done to, or about, the Kurds. Long living in the poor mountains of the central Middle East, Kurdish life was rarely above sustenance. Even when a powerful Kurdish leader emerged, he didn’t do much for his Kurdish compatriots. When the Ottomans conquered them, they were an afterthought. During World War I, they were so few that even the imperial planners in London and Paris didn’t give them much thought.

The twentieth century’s technological changes affected Kurdistan too. What was once an enclave of mountain dwellers barely above starvation managed to swell to nearly 32 million in Turkey, Syria, Iraq and Iran. There are 74 million people in Turkey; of them, sixteen million are Kurds. Under the most generous of troop to citizen ratios at 2.2, Turkey still needs over 35,000 troops in Turkish Kurdistan to pacify it.

You’d be hard pressed to find any nation state willing to surrender that amount of territory
And this is generous. There are powerful and organized Kurdish forces that despise Ankara. A number closer to 100,000 or more seems a better ratio, considering that rebellious Kurds in the Kurdistan Workers’ Party, or PKK, are experienced and able.

Ankara could try the genocide trick again, but to do would almost certainly get it kicked out of NATO, if not invaded by NATO itself. To ethnically cleanse the Kurds might provoke the West the same way. So Turkey is stuck with the Kurds unless it’s willing to lose its eastern regions to an independence Kurdistan. But no nation state sheds territory willingly. Since nations tie themselves to not just their cultures but also to their land, nations that lose territory become unstable and hard to govern.

Which brings us to why Turkey has suddenly gone into Syria

That Turkey means to send a message to the Kurds is clear enough, but there are wider ambitions as well. To cow the Kurds is to do more than militarily defeat them, because, as we have seen, Turkey cannot rid itself of its Kurds and so long as the Kurds hate Ankara’s rule they will be a huge drain on its geopolitical resources better used elsewhere. So instead, Turkey must do two things: it must coax Kurdish moderates to subsume themselves into Turkish political life and it must destroy the hopes of Kurdish separatists.

Getting Kurdish moderates to become part of Turkey’s political fabric is a long, slow process that will go nowhere so long as separatists think they have a shot at getting a free Kurdistan.

And there is only one way Kurdistan can emerge onto the map: if the region is in sustained, continuous chaos.

Kurdish separatism threatens to go septic and thus the Turks have rolled into action. They mean to establish order in Syria that prevents another Iraq-style de facto partition into a Kurdish statelet and an Arab rest. By moving directly into Syria with both its own troops and its direct proxies, Turkey secures its core geopolitical interest.

But by putting NATO troops into Syria, Turkey also will force the United States and Russia to solve the conflict sooner than they might otherwise. The more Turkish troops take territory in Syria, the greater the pressure will be to end the civil war that has given breath to the Rojava Kurds of Syria. Both Moscow and Washington want to avoid a direct war in Syria at all costs. Turkish troops at risk of Russian bombardment makes a clash between NATO and Russia far more likely.

But Turkey cannot also attack Assadist troops directly or it could risk being abandoned by its increasingly lukewarm NATO allies. The easiest target is the Islamic State. IS has long been a tool of Turkish policy: Turkey turned a blind eye as its recruits, arms and oil smugglers crossed the border so long as IS kept its war contained to Syria and was a good balance to the Kurds. But IS has ambitions beyond that and so lashed out at Turkey, hoping to provoke strife between the government and Turkish Kurds. Now IS serve a new purpose for Turkey: giving it a excuse to invade Syria.

And all goes toward a wider aim: building Turkish influence in Syria to counter Iranian and Russian influence there

The Turks cannot hope to rule Syria as they once did. What they can do is impress proxies into securing their core interests.

Up until now Turkey hasn’t convinced many in Syria to consider what Turkey wants, but with the Turkish army on the move that calculation will change.

It is high risk, high reward. Turkey could be bogged down in the same civil war that neither Iran nor Russia, with all their might, have been unable to win. It could run into Russian or Iranian forces and risk a much larger geopolitical crisis, much as it did after it shot down a Russian jet last fall.

But it could also halt the steady slide of Kurdish separatism that threatens Turkey itself. It could reverse the growth of Russian power into the Eastern Mediterranean. It could reduce Iranian power along its border. On a long enough timeline, its proxies could gain the upper hand in Syria, turning what was once a major thorn in Ankara’s side into a useful bastion of Turkish power. It does not mean conquest; those days are over. It means impressing enough Kurds to give up secessionism and enough Syrians to cooperate with Turkey once the civil war ends.

The outcome of the Syrian Civil War will define the trajectory of Turkish power for years, if not decades. Should it achieve its goals as the embers die out, Turkey could regain much of its lost status and bring into its orbit Sunni states desperate for some other lodestar beyond backward Saudi Arabia to follow. Jordan, Egypt, Libya and Lebanon are all obvious candidates for a renewed era of Turkish domination of the Eastern Mediterranean.

But first, Turkey must win on the battlefield in Syria. The coming weeks will show what they are capable of.

This story first appeared at Geopolitics Made Super, August 31, 2016.