Turkish-backed rebels fought with U.S.-backed Kurdish fighters north of Aleppo on Monday. It’s not an isolated incident—there has been sporadic fighting between the two groups for months—and the violence feels like a preview of the next phase of the war in Syria.

Since the outbreak of the war, Syrian Kurds—long marginalized and discriminated against under Bashar al-Assad’s government—have carved out a semi-autonomous region in Northern Syria. The main Kurdish armed group, known as the People’s Protection Units, or YPG, has also emerged as the main and most effective U.S. ally in the fight against ISIS. (Technically, the U.S. is working with the Syrian Democratic Forces—an umbrella group that includes both Arab and Kurdish forces—but the YPG is widely acknowledged as the dominant partner in that arrangement.)

This is a problem for Turkey, which views the YPG as a wing of the PKK, the Turkey-based Kurdish separatist group that has fought against the government for decades. There’s some justification for this—the two groups have cooperated in the past and are both devoted followers of the political philosophies of PKK founder Abdullah Ocalan—though the YPG claims to operate independently. Last summer, Turkey began a direct military intervention in Syria, using both its own troops and sympathetic rebels, in an effort to contain Kurdish gains. Earlier this month, some Syrian rebels said they were preparing to join with Turkish forces in an offensive against the Kurds. The presence of U.S. forces on the ground and political pressure on Turkey from Washington may be the only reason this offensive hasn’t already happened.

All of this is a headache for the United States, which is also allied with Turkey. And it gets even more complicated: Whenever ISIS is routed from its capital in Raqqa, the Syrian Kurds, who led the operation to capture the city, are going to want to preserve the political gains they’ve made and will expect U.S. support. Unlike other rebel groups, the Syrian Kurds are not calling for al-Assad’s overthrow, which has allowed for an uneasy truce between them and Syrian government forces. And unlike their Iraqi Kurdish counterparts, they’re not calling for full independence, either. Rather, in accordance with Ocalan’s semi-anarchist ideas, they’re calling for a loose confederation that allows them substantial autonomy. Despite recent Russian-brokered talks between the two sides, Assad seems unlikely to agree to Kurdish demands, and clashes between the regime and the YPG seem inevitable. In a recent article for Foreign Affairs, analyst Sam Heller argued that the U.S.-Kurdish partnership against ISIS, while probably the best option available, has created a condition in which the U.S. may have to maintain a presence in Syria indefinitely in order to prevent the situation from developing back into chaos that could allow ISIS to regroup.

Of course, the fate of the Syrian Kurds is just one big question that could lead to future tensions as the conflict transforms. The fight against ISIS has required delicate alliance-building, at times contradictory partnerships, and a whole lot of tricky wrangling of reluctant participants. It may end up looking simple compared with what comes next.