Ankara's relationship with Washington might face a new test with Turkey's President Recep Tayyip Erdogan announcing that Turkish forces will enter the northern Syrian town of Manbij to clear the Syrian Kurdish Democratic Union Party (PYD) and its affiliated militants (YPG) on December 14. Given that Manbij has been under U.S. supervision for years and the closeness of ties between PYD and the U.S., Turkey's threat to enter Manbij might provoke the hostilities between U.S. forces in the town and erode bilateral ties between the two states.

The Manbij area comes as a sensitive issue between Turkey and the U.S. The relations between the two countries have become more tense during the past years. This distrust could be attributed to several major issues, ranging from their different attitudes to Washington's sanctions against Iran, Washington's rejection of extraditing the Turkish preacher Fethullah Gulen, and the persist suspicion from Turkey of U.S.'s role in the failed military coup in 2016.

The most pressing issue for the two states is the Syrian Kurdish issue. The two nations have been at odds over Syria, especially its northern region for years. The U.S. has supported and backed the PYD in northern Syria to fight against ISIL and other local Islamic extremists, while Turkey considers the PYD and YPG terrorist organizations that are closely linked to Turkey's outlawed group, Turkish Kurdish Workers' Party (PKK).

Manbij is a major town in northern Syria and has nearly 10,000 residents before civil war erupted in Syria in 2011. The Islamic extremists, especially ISIL, captured Manbij in 2014. The town soon became an important hub for smuggling and transporting weapons. In 2016 the PYD dominated Syria Democratic Forces (SDF) launched an offensive and retook Manbij, and expelled ISIL militants from the town. Under the supervision of PYD and the protection of YPG and SDF, Manbij today is a self-administered town governed by a self-organized local city council.

To prevent the resurgence of local Islamic extremists in both Manbij and nearby areas, the U.S. offered support and assistance to YPG and SDF in Manbij. With the help of the U.S., YPG and SDF established training centers in Manbij to recruit and train local militants to fight against ISIL militants in northern and eastern Syria. Meanwhile, the U.S. also sent its special forces and military convoys to Manbij to help stabilize the local order.  

Turkey also attaches high importance to Manbij and has been maintaining that the Syrian Kurdish militant groups in northern Syria should be eliminated. On the one hand, Turkey has supported certain Syrian rebel groups, such as the Free Syria Army (FSA) and Tahrir al-Sham, which are stationed primarily in Idlib province and Aleppo province in northern Syria. In early 2018, Turkish forces and the Syrian rebels they backed launched a massive offensive against the enclave of Afrin in northern Syria, and Erdogan once promised to eliminate all of the Kurdish militants in northern Syria to establish a buffer zone.

To deter Turkey's military offensive, especially the possible offensive against PYD and SDF militants in East Bank of Euphrates River, the U.S. and Turkey reached an agreement in mid-2018 and began joint patrols around Manbij. For Turkey, the U.S. and its military presence in northern Syria, especially in East Bank of Euphrates River, have become a major source of protection for Syrian Kurds. With the support and help from the U.S., the SDF and YPG gradually expanded in northern Syria. Last week, SDF spearheaded by YPG retook the town of Hajin in eastern Syria, which had been under ISIL control for years.

Although the East bank of Euphrates River of northern Syria is under the protection of the U.S., the future status of Manbij is still unclear. According to the deal reached by the U.S. and Turkey in November, the YPG members in Manbij should be withdrawn to the eastern bank of Euphrates River. After Erdogan's promise of entering Manbij, a Turkish soldier was killed in Afrin by a YPG member on December 13. It is very likely that the anger of Turkey and the Turkish soldier's death could cause a new military offensive.

Although the bilateral tension between Ankara and Washington is alleviated by Turkey's release of American pastor Andrew Brunson in October and is further improved by the U.S. lifting sanctions against Iran and their close communication after Khashoggi's murder, the distrust remains. Turkey's ambition of launching a new offensive suggests that the deeply rooted friction and split are still there.

However, Turkey needs to reconsider its military offensive in northern Syria. Turkey is still experiencing economic hardship at home with further downward pressure. To Erdogan and his AKP-led government, it is the economy rather than the foreign affairs in Manbij that determines their political dominance at home. Therefore, it might not be wise for Turkey to launch a premature military offensive in Manbij, where U.S. military forces are deployed.