For a start, Trump’s claim that ISIS has been defeated is inaccurate. ISIS continues to hold a small but critical pocket of land consisting of a handful of towns along the Euphrates River and the Syrian-Iraqi borders. The campaign to expel the militants from their last stronghold is expected to take a month or two—although the announcement could mean a more aggressive effort that would increase civilian casualties.
Even after the defeat of ISIS in this pocket, the fight will hardly be won. The U.S.-led campaign created new dynamics on the ground, which only the U.S. can now address. For example, U.S. reliance on the Kurdish militia known as the YPG angered Turkey, which views it as an extension of the PKK, the insurgent group that it has been battling in-country for four decades. As the YPG expanded its influence during the fight against ISIS, Ankara shifted its priorities in Syria from backing the opposition in its fight against the regime to aligning with Russia to fight the Kurds.
The timing of Trump’s decision is especially destabilizing, because various parties in the conflict have positioned themselves to attack the areas under YPG control as the campaign against ISIS draws to a close. The end of the campaign means that the U.S. has to work harder, not the opposite, to ensure a peaceful settlement in eastern Syria. Such political support is a fundamental part of finishing the job against ISIS, and this cannot happen without the leverage Washington currently has by being present in Syria.
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So far, the U.S.’s presence has prevented Turkey from clashing with the YPG in eastern Syria, as it did in northwestern Syria with Russian approval. Washington does not have the luxury to limit its goal to defeating ISIS militarily and then pulling out. If it simply leaves, new wars will almost certainly be reignited between Turkey and the YPG, the Arabs and the Kurds, and the opposition and the YPG.
Much of the response to Trump’s decision focused on how the move could enable ISIS to return, but the immediate consequences will extend beyond ISIS: The Syrian conflict, after months of quiet, could be rekindled. The end of fighting in most of the country over the past year was possible because of delicate agreements brokered by Russia and various parties and because of the U.S. commitment to the Syrian Democratic Forces (the alliance of Kurdish, Arab, and Assyrian militias led by the YPG). But the peace is still profoundly fragile.
The nightmarish scenario of a return to all-out violence throughout Syria could directly undermine U.S. interests, including by provoking new waves of refugees and emboldening extremists. At this moment, the forces most active against the Syrian regime are predominantly those linked to ISIS and al-Qaeda, because the moderates have already struck deals with Damascus and Moscow. This means that hard-liners will be well positioned to inherit what remains of the rebellion against the regime.