“We must always have the backs of our allies, if we expect them to have our back,” tweeted former U.S. ambassador to the United Nations Nikki Haley. Sen. Marco Rubio (R-Fla.) called the decision “morally repugnant,” while Sen. Chris Murphy (D-Conn.) attacked it as “positively sinister.”

To be sure, the People’s Protection Units (YPG) members who fought valiantly against the Islamic State deserve American gratitude, and Trump should have strongly discouraged Turkey from attacking them. But the situation is complicated. The policy of assisting a faction of Syrian Kurds, the YPG, to fight the Islamic State has been a ticking time bomb since it began under the Obama administration, in 2014.

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The short-term imperative to combat the militant group, which is also known as ISIS, created a strategic contradiction with foreseeable consequences that are now on painful display. Turkey, a NATO member, never accepted U.S. support for the group, which is directly linked to a terrorist organization that has long fought an insurgency against the Turkish state. Nor did every politician now criticizing Trump — whose decision-making has been characteristically chaotic — think this alliance was wise in the first place. The main surprise here is that U.S. diplomats and military officers made this untenable situation work as long as it did.

President Barack Obama did not align the United States with the YPG with any enthusiasm. He was reluctant to intervene in the Syrian civil war. But he was alarmed by the emergence of the Islamic State, which he believed threatened the United States and its regional allies. The United States launched an air campaign in September 2014 to prevent the group from seizing control of the Kurdish town of Kobane near the Turkish border, later airdropping supplies to YPG forces defending the city. Rather than putting American boots on the ground, the United States wanted to partner with local fighters to defeat the Islamic State. Turkey objected to continued U.S. cooperation with the YPG and proposed an alternative Sunni force, but the United States did not believe those fighters were sufficiently “moderate” and suspected that they (like Turkish leader Recep Tayyip Erdogan) were more focused on overthrowing Bashar al-Assad’s regime than eliminating the Islamic State. Erdogan also wanted a buffer zone to keep YPG forces away from Turkey’s border, one that would also provide a haven for the refugees flooding his country, a proposal Obama did not support. Faced with an immediate risk and no good options, U.S. Central Command began assisting the YPG. It quickly developed an affinity to its brothers-in-arms, even wearing YPG patches in solidarity — until Turkey objected.

Erdogan opposed this partnership because of the YPG’s links to Turkey’s Kurdistan Workers’ Party (PKK), which is designated as a terrorist organization by both Ankara and Washington. The PKK’s armed struggle against the Turkish state for Kurdish rights has resulted in more than 40,000 deaths, including several bombings in Istanbul and Ankara that killed dozens of civilians in 2016 alone. The United States does not classify the YPG as a terrorist entity — a legal distinction that allowed the Obama administration to justify its cooperation with the group. Both groups operate under the same command structure, and fighters move freely between them. Indeed, the YPG is militarily effective in part because of its members’ experience in Turkey. Links on the ground are clear: After the YPG defeated the Islamic State in the Syrian city of Raqqa, fighters unfurled banners showing Abdullah Ocalan, the imprisoned founder of the PKK.

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Turkish officials were unimpressed by U.S. efforts to make the YPG more palatable by adding Syrian Arab fighters and creating a new umbrella organization called the Syrian Democratic Forces (SDF). They note that Gen. Raymond Thomas, former head of U.S. Special Operations Command, has said publicly that he asked the YPG to “change your brand” and described the inclusion of “democratic” in the group’s name as a “stroke of brilliance.”

Sen. Lindsey O. Graham (R-S.C.), among the harshest critics of Trump’s decision this week, was initially sympathetic to Turkish concerns and called the partnership with the Syrian Kurds “the dumbest idea in the world” in an April 2016 Senate hearing, given the PKK connection.

The YPG’s decision to fight the Islamic State was not made to help the United States. The group had its own reasons — in addition to defending its territory, the YPG sought to connect three northern Syrian cantons into a single autonomous region dominated by Kurds (similar to the Iraqi region of Kurdistan, which is led by a different faction of Kurds). This territory, however, borders the southeastern part of Turkey, where much of its Kurdish population lives. Turkey worried that the United States’ strengthening of the YPG in Syria could encourage the PKK to renew conflict at home. A 30-month cease-fire and peace talks that Erdogan had begun with the PKK broke down in July 2015, partly because of spillover from the conflict in Syria. (Some Syrian Arabs resented the YPG’s growing presence in areas that were not historically Kurdish and accused the group in summer 2015 of forcibly displacing Arab villagers.)

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Although Washington sought to assuage Ankara, it made promises that proved hard to keep. When the Obama administration asked the YPG in spring 2016 to cross the Euphrates River to clear the predominantly Arab town of Manbij of Islamic State fighters, it was acting against Erdogan’s express wish that the YPG remain east of the river — a natural boundary that separated the cantons. The United States publicly promised Turkey the fighters would leave afterward, yet some still remain. When the Trump administration armed the YPG for the Raqqa operation, which began in June 2017, it promised to collect the weapons afterward. A U.S. official described this idea as “asinine;” predictably, it didn’t fully happen.

In its eagerness to defeat the Islamic State, the United States failed to address the long-term consequence of creating a Syrian Kurdish region that was flatly unacceptable to Turkey. And given that the United States described its cooperation with the YPG as “temporary, transactional, and tactical” — as a senior State Department official put it — it is hardly surprising that Erdogan pressed for an end date.

Turkey has conducted military operations in Syria before, to create a buffer zone along the border, prevent the establishment of an autonomous Kurdish entity, and provide a space to return refugees. In August 2016, the Turkish military launched Operation Euphrates Shield to clear the Islamic State from its border (from Jarabulus on the Euphrates River to Azaz in the west, near the Kurdish enclave of Afrin) and block the Syrian Kurds from creating a contiguous region. In January 2018, Operation Olive Branch directly targeted YPG fighters in Afrin — who were working with Russian rather than U.S. military advisers. The U.S. reaction was restrained, acknowledging Turkish security concerns yet urging it to act in a proportionate and measured way.

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To prevent Erdogan from further military action, senior U.S. officials worked with their Turkish counterparts in recent months on plans to establish a limited buffer zone in Syria, along part of the Turkish border. The two sides conducted joint patrols and established a joint operations center. The United States also asked the YPG to pull back forces and remove fortifications along the border, which now leaves them defenseless. Yet Erdogan grew impatient with the failure to agree on details and demanded a 20-mile deep and 300-mile wide corridor — far larger than the narrow strip envisioned by Washington.

Trump’s hasty decision to withdraw U.S. advisers from the Syrian border, and at least tacitly approve a Turkish military operation, was sloppy and cruel. The lack of a coherent policy process and garbled messaging made a dangerous situation even worse. Renewed fighting will harm civilians in a now peaceful part of a war-torn country, enable the Islamic State to regroup, and empower Russia and Iran, who are backing the Assad regime and hungry for more influence.

Ending the alliance with the YPG may be inevitable, but Trump’s critics are right that the United States can’t simply walk away without tarnishing its reputation as a reliable partner. With Turkey’s Operation Peace Spring now underway, the United States should insist it be limited in scope and not target major Kurdish-controlled towns, that it ensure that Islamic State prisoners held by the YPG are not released, and that it not provoke a humanitarian crisis among civilians.

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The United States should continue supporting efforts to find a political solution to the broader conflict in Syria, including decisions about long-term governance and security arrangements. The YPG produced expedient results in one corner of Syria, but it was never expected to liberate the entire country. A likely outcome of a Turkish incursion is that the YPG makes a deal with the Syrian regime. Throughout the conflict, the YPG never joined opposition groups seeking to overthrow the regime, which, in turn, did not conduct extensive operations in areas where the YPG was based.