The full consequences of President Trump’s decision on October 6 to withdraw American troops and give Turkish president Recep Tayyip Erdoğan a green light to invade northeast Syria are not yet clear. Erdoğan claimed that he wanted to create a twenty-mile buffer zone in which perhaps one million Syrian refugees living in Turkey could be resettled, but he may have had the ambition of turning all of northeast Syria over to the Islamists whom Turkey had sponsored in western Syria during the country’s civil war and who were largely defeated there.

Thanks to deft Russian diplomacy, that ambition—which could have reignited the Syrian civil war just as it was winding down—appears to have been largely thwarted. But it is hard to imagine a more calamitous outcome for the United States, the Kurds, NATO, and possibly Turkey itself. Turkey is unlikely to accomplish its stated objective of eliminating Kurdish control of the border zone, while its invasion threatens to rupture relations with the West and lead to sanctions that would further shrink a contracting economy. If it comes to a prolonged fight against the Kurds, Turkey cannot rely on the undisciplined Syrian proxies that it has used so far, meaning large numbers of regular Turkish army troops will be engaged against a skilled and determined adversary. Costly battles with a conscript army are rarely popular at home.

Before the Turkish offensive, which began on October 9, Syria was essentially divided along the Euphrates River (see map below). To the west, the Syrian government had mostly overcome a disparate group of foes, including Islamists, Turkish proxies, warlords, and a small number of Western-oriented democrats. Only Idlib governorate in the northwest remained outside the control of Syrian president Bashar al-Assad.

To the east of the Euphrates, the Kurdish-led Syrian Democratic Forces (SDF) controlled almost one third of the country. With US air support and the assistance of around three thousand US special forces and CIA operatives, the SDF had prevailed in a five-year battle with ISIS, whose “caliphate” had at its peak in late 2014 controlled more of Syria than either the government or the Kurds. During that time, the SDF lost 11,000 fighters, while the US sustained five combat casualties.

By October 13, under the terms of a Russian-brokered deal between the Syrian government and the SDF, Assad’s forces had moved into the major population centers in northeast Syria to support the Kurds and as a first step to reestablishing Syrian government authority in the region. Turkish artillery fired on an American base, and the US scrambled to pull out troops and diplomats. In his public statements and tweets, Trump alternated between defending his green light on the grounds that the Kurds were “not angels” and didn’t fight on the US…