Secretary of State Rex Tillerson faces perhaps the most difficult stop of his Mideast trip in Turkey, where President Recep Tayyip Erdogan has doubled down on anti-American rhetoric by appearing to threaten US troops in Syria. Turkey has bristled at the US military’s alliance with Syrian Kurds to fight ISIS, fearing the Syrian group will one day help Turkish Kurds make a push for independence. Turkey has sent its troops to attack a Kurdish enclave in Syria and it threatens to push into a town where US forces train Kurds and others, leading the Pentagon to warn Ankara away. “It is clear that those who say, ‘We will respond aggressively if you hit us,’ have never experienced an Ottoman slap,” Erdogan said in response on Tuesday, according to news reports. Erdogan’s comment was just the latest salvo from an ally who has accused the US of complicity in a coup attempt, eroded democratic norms within the country, jailed Americans on flimsy grounds and detained US consulate workers. Beyond threats about a clash with US forces, Turkey’s move to attack the Kurdish enclave of Afrin in northwest Syria could undermine the coalition fight against ISIS, as Kurdish YPG fighters who are helping the US rout the terrorist group in the east have begun leaving to help their compatriots in Afrin. At the State Department, spokesman Heather Nauert brushed off Erdogan’s “slap,” saying, “We’re used to that kind of rhetoric.” On the road in Jordan, Tillerson calmly stressed the long game. “Turkey is still an important NATO ally of the United States, they’re still a very important partner in the region for us, and we need to find a way to continue to work in the same direction,” Tillerson told reporters in Amman. He stressed that the two countries still share goals for Syria and the US recognizes Turkey’s security concerns. “So we hope to have talks about how we can work cooperatively to lessen those threats to Turkey but ultimately achieve the objective in Syria, which is the full and enduring defeat of ISIS, the de-escalation of violence in Syria,” and progress with peace talks that stabilize Syria, Tillerson said. But Tillerson “has a tough set of meetings and a tough job ahead of him when he’s in Ankara,” said Ross Wilson, who was the US ambassador to Turkey from 2005 to 2008. “It’s a very fraught time,” Wilson said. “There’s an immense amount of tension in US-Turkey relations.” Luke Coffey, director of the Allison Center for Foreign Policy Studies at the Heritage Foundation, noted that “a lot of [Erdogan’s] rhetoric is for a domestic audience, and it’s not new.” Repaired or completely damaged Turkish bitterness escalated after the US aligned with the YPG in 2016, which proved to be the most effective fighting force on the ground against ISIS, said Wilson, who’s now a distinguished fellow at the Atlantic Council. Wilson added that the “decisions to cooperate with the YPG have been military ones, not strategic ones about calibrating the relationship with Turkey. It’s almost transactional. This is what we’ve had to do to combat ISIS.” The White House, signaling its sensitivity to Turkey’s “legitimate security concerns,” sent National Security Adviser H.R. McMaster to Istanbul on Sunday to underscore that message over the weekend, but his Turkish counterparts weren’t receptive. After McMaster’s meetings, Foreign Minister Mevlut Cavusoglu warned that Turkish-US ties had hit a critical point, and would either be repaired or completely damaged. A US announcement in mid-January that it would form a 30,000-strong Kurdish-led border force in northeastern Syria sparked this heightened period of US-Turkey tension. Erdogan accused the US of “building an army of terror” on his border and, in extraordinary language from a fellow NATO ally, threatened to “drown” the US-backed force. While the US backed away from the plan and Tillerson said it had been “misportrayed, misdescribed,” tensions escalated as Turkey moved into Afrin. It has since threatened to push further to Manbij, where US Special Operations Forces are supporting Syrian Kurds. The Turkish government could retaliate by imposing greater costs on the US military for choosing to work with the YPG, Wilson said, including limited access to its Incirlik air base, which has made operations for the coalition against ISIS cheaper and easier. “The fact that the Turks have not chosen to go down that road so far,” Wilson said, might indicate that “it should be possible to calm things down. The mere fact that he is going to Turkey is helpful in and of itself.” A State Department official, speaking on background, said Tillerson would deliver the same message privately to Erdogan that the US has been stressing publicly. “We are urging them to show restraint in their operations in Afrin,” the official said, “and to show restraint further along the line across the border in northern Syria.” While the US means to work with Turkey to address its security concerns, the official said the US goal is, “above all else, keeping everything focused on the defeat-ISIS fight, which is not over.” A second State Department official briefing reporters at the outset of Tillerson’s trip said that, “the rhetoric is hot, the Turks are angry and this is a difficult time to do business, but it’s our belief that there are still some very fundamental underlying shared interests that we have with the Turks, in terms of stability in Syria, defeating ISIS, fighting the PKK and broader questions of regional balance of power, where the Turks play an important role.” “At times like this, engagement is all the more important, but … it’s going to be a difficult conversation,” the official said.