Analysis of President Donald Trump’s December 19 announcement that U.S. military forces would withdraw from Syria within 30 days has rightly focused on the potential adverse consequences for the unfinished campaign to defeat the Islamic State (ISIS), and on identifying the geopolitical winners and losers. Syrian President Bashar al-Assad, who was already in a strong position, is finally free of any meaningful threat from Washington. His principal backers, Russia and Iran, have gained prestige and regional influence. Meanwhile, the closest U.S. partners in Syria – the Kurds, but also Britain and France – have been left holding the bag.

But this is hardly the final round in Syria’s civil war, and U.S. policymakers and others would be well-served to think through what will happen when the strategic map resets. Of course, the United States was never the dominant actor in Syria, and its influence routinely has been overstated. Nevertheless, over the past seven years, big Syria policy decisions – good, bad, and ugly – have profoundly impacted the course of the war. If implemented, Trump’s latest decision will be no different. Russia and Turkey, the two most important external actors remaining in Syria, both stand to gain from the U.S. withdrawal, but both also have reason to be anxious over precisely how the resulting vacuum will be filled. How are Russia and Turkey likely to react, and what might be the consequences, positive or negative?

For several months, the war in Syria has effectively been frozen. After recapturing pockets of insurgent control in the Damascus suburbs and along the Jordanian border earlier this year, Assad’s military campaign has ground to a halt. Instead of the Assad victory some predicted, a series of ad hoc, internationally-brokered agreements have solidified lines of control and prevented further advances by any side. The result has been a de facto partition of the country into three clearly delineated zones of influence in the west, northwest, and east – backed, respectively, by Russia and Iran (west), Turkey (northwest), and the United States (east). While this arrangement has dramatically diminished the bloodshed, it has not satisfied the strategic objectives of any of the major parties and therefore remains highly unstable.

Two weeks ago, Turkish President Recep Tayip Erdogan forced the issue, stating that Turkey was ready to invade the U.S.-backed zone to remove from its border Kurdish military forces aligned with the Kurdistan Worker’s Party (PKK), which Ankara has been fighting for more than thirty years. After playing an instrumental role in the battle of Kobani in October 2014, these forces emerged as the key U.S. partner in the fight against ISIS in Syria. Turkey views their growing military strength and political legitimacy as an acute threat to its national security – a point Erdogan has made repeatedly to both President Obama and President Trump. Erdogan’s promise that a Turkish invasion would help to “eradicate whatever’s left of ISIS” is a ruse: Turkey’s objective is to weaken the Kurds, not ISIS, and in any case the remaining pocket of ISIS control is more than a hundred miles from the Turkish border, all through Kurdish-controlled territory. Nevertheless, Trump appears to have taken this promise seriously.

So long as U.S. forces remain in Syria, Erdogan’s military plans are a very risky proposition. But a U.S. withdrawal upsets the fragile equilibrium, opening the door for Turkey and leaving the Kurds exposed. The Kurds cannot maintain control over the territory they currently hold absent international security guarantees: Kurdish lines are too long, their control over traditionally Arab areas too tenuous, and they have no air force to counter attacks from either Turkey or the Syrian government. France has publicly expressed continued support, but it is unclear how a relatively small French contingent can operate without the larger framework of U.S. logistics and intelligence, or whether a French presence would be sufficient to deter an attack. If and when U.S. forces leave, Kurdish-controlled territory is likely to contract sharply.

The main question is who will claim the spoils: Turkey or the Syrian government (with Russian and Iranian help)? There are two basic scenarios. In one, Turkey launches an attack on Kurdish forces east of the Euphrates and seeks to expand its own sphere of influence on the Syrian side of the border. If successful, this would neutralize the Kurdish threat and create another buffer zone in which Syrian refugees might ultimately be able to return, analogous to the Euphrates Shield area in northwest Syria. In the second scenario, the Kurds cut a deal with Damascus that returns eastern Syria to government control, in return for some degree of Kurdish autonomy and protection from Turkey. Of course, these scenarios are not mutually exclusive: Turkey could proceed with a preliminary operation against Manbij, on the Kurds’ northwestern flank, at the same time that the Syrian military and its allies attempt to retake Tabqa Dam or oil and gas infrastructure in Deir ez Zor province, further south.

In any scenario, the scramble for territory is likely to sharply increase tensions between Russia and Turkey. While Turkish threats work to Damascus’ advantage, since they increase its leverage in negotiations with the Kurds, neither the Syrian government nor Russia want to see a fresh Turkish invasion. This would undermine Assad’s core objective of reclaiming “every inch” of Syrian territory, create a new safe haven for Turkish-backed Syrian forces that continue to oppose Assad’s rule, and potentially deny Damascus access to energy and agricultural resources that it desperately needs to fund reconstruction. Russia is therefore likely to seek to accelerate talks between the Syrian government and the Kurds, while offering Turkey assurances that its interests will be protected and, crucially, pressing Turkey not to press further into eastern Syria. Russia has cards to play here: in addition to Turkey’s dependence on Russian natural gas, Russia serves as the guarantor of the October agreement in Idlib that forestalled a Syrian government offensive which could send more than a million new refugees into southern Turkey. In Idlib, Russian and Turkish interests aligned; in the east they do not.

At the same time, however, Erdogan appears to have a green light from Washington, and in any case there are sound strategic reasons for Turkey to proceed, even in the face of protests from Moscow. The departure of U.S. military forces from eastern Syria is not an unambiguously positive development for Ankara. Turkey has lost the ability to play the United States and Russia off one another, and should the Syrian government and the Kurds reach an accommodation, there is every reason to suspect they will soon find common cause against Turkey, as they have many times in the past. This is likely to begin with Assad’s support for a Kurdish offensive to retake Afrin, which Turkey seized in March, and could develop into broader cooperation to harass and ultimately expel Turkey from all of northern Syria. There is no question that expanding Turkish operations into the east carries significant risks, which is probably one of the main reasons Erdogan has delayed his plans. But sitting tight while Turkey’s adversaries in Syria unite is not terribly appealing, either. By proceeding, Erdogan might be able to deal the Kurds a decisive blow, while keeping attention focused on the east (rather than the northwest) and giving himself more chips to trade in when he needs them.

Whatever happens, with the United States out of Syria it is hard to see how Russia and Turkey do not rapidly find themselves at loggerheads – either because Russia is unable to protect Turkey’s interests in Syria, or because a Turkish invasion undermines Russia’s efforts to bring the war to a successful conclusion. This would be welcome in so far as it might bring Turkey back into closer alignment with the United States, but an unintended confrontation between the two countries inside Syria or a major clash between Russian and Turkish proxies could easily spiral out of control. In the most extreme case, this could result in Turkey invoking collective self-defense under article 5 of the NATO treaty, which would bring the United States back to Syria in a very different, and far more costly, role. But even absent that, a fresh escalation of the war would be a disaster – for Syria, the region, and international efforts to defeat ISIS.

The challenge of preventing an unwanted escalation is of course made infinitely harder when consequential decisions are made on the fly, without adequate consultation or planning. A responsible withdrawal from Syria requires a diplomatic strategy to manage what comes next.

Photo: Turkish-backed Syrian fighters train in a camp in the Aleppo countryside, northern Syria, on December 16, 2018 (Aref Tammawi/AFP/Getty Images)