The United States may have more leverage. The Paris and Brussels attacks have raised the pressure on President Obama to destroy the ISIS sanctuary in Syria and Iraq, and it is clear that Turkey’s Kurdish conflict is putting this effort at risk by dividing the administration’s two key allies. “What’s concerning is that while Turkey has every right to fight the P.K.K., it’s increasingly becoming a conflict pitting Turkey against the Kurds,” one administration official told me. “That jeopardizes the credibility of even our friends among the Kurds, including Iraqi Kurds.” American officials have tried to keep their Kurdish allies in Syria from unnecessarily provoking the Turks. They say they are not aware of any P.Y.D. fighters or weaponry being transferred north to be used against the Turkish military. The Turks dispute that claim and insist that American weapons sent for use against ISIS have been passed northward and used by the P.K.K. (The shoulder-fired missile shown in the recent online video has prompted widespread speculation about a weapons trail from Syria.) The Obama administration officials I spoke with said they believed that if a peace deal is reached, the Turks will eventually become reconciled to a Kurdish statelet in Syria, just as they agreed to (and even helped midwife) the birth of Iraqi Kurdistan a decade ago. Perhaps. But the Turks say the two situations cannot be compared. In mid-March, the Syrian Kurds took another step toward independence, voting to establish a self-governing federal region. The announcement prompted angry rebuttals from the Turkish and Syrian governments and Arab opposition factions fighting in Syria.

Meanwhile, the Americans are trying to find Arab allies who could substitute for the Kurds in a push to conquer Raqqa, the Islamic State’s capital. But they are hobbled by Turkey’s insistence on treating not just the P.Y.D. but also anyone who has worked with it, including Arabs, as terrorists. The fact that Erdogan’s government has itself supported hard-line Islamist militias in Syria — including some with ties to Al Qaeda — only adds to the American frustration. The Obama administration appears to be forging ahead with its plans, despite Turkish complaints: In late April, Obama announced that an additional 250 Special Operations troops were on their way to northeastern Syria to work with the Kurdish-led force there. American officials say they have little choice; the Turks have provided no viable alternatives for an assault on Raqqa. For Erdogan, fighting ISIS remains secondary to ousting President Bashar al-Assad of Syria, whom he blames for the region’s turmoil.

Erdogan’s visit to Washington in late March did nothing to address these tensions. The visit was for nuclear-security talks, but Erdogan reportedly lashed out angrily at American policy during an off-the-record dinner with former government officials and academics, and complained repeatedly about the American refusal to treat the P.Y.D. as a terrorist group. During a public speech at the Brookings Institution, his security guards got into a nasty melee outside the building, shoving and shouting at journalists, protesters and Brookings personnel. The guards ordered a prominent Turkish journalist, Amberin Zaman, to leave, calling her a “P.K.K. whore”; security staff members had to stop the guards from removing other journalists from inside the auditorium. In video clips, beefy Turkish guards in dark suits and sunglasses can be seen shouting at pro-Kurdish protesters and being restrained by police officers. Turkey’s Kurdish problem seemed, for a moment, to have spread all the way to Massachusetts Avenue.

A few days after my last visit to Nusaybin in March, the authorities announced a new military “curfew,” and thousands of civilians began streaming out of the town. Within days, images began appearing on Twitter of tanks firing into the same buildings where I had sat chatting with young insurgents, and smoke rising from the city. Many of the photographs were taken by sympathetic Syrian Kurds from just over the border in Rojava, where they had a good view of the fighting. The Turkish media reported soldiers killed by bombs and snipers. The activist who had taken me to Nusaybin sent me a set of color sketches she made on her iPad, showing tanks, fighters with guns, bloodied bodies, dead children.

I thought about Omer Aydin, the P.K.K. commander who told me his life story. Most of the fighters I met refused to say much about their feelings; they retreated behind a stoical mask and repeated the same P.K.K. talking points. Not Aydin. “I can’t forget the face of this soldier I killed,” he said at one point. “At the end, they are human, too.” Aydin seemed a little defensive about the justice of his cause, as if he understood that he and his fellow rebels were putting the lives of ordinary people at risk. A few months earlier, he told me, he’d been on the lookout for Turkish snipers when he walked past a mother holding a baby who was crying uncontrollably. The sight unnerved him for a moment. “I passed, and when he saw my weapon, he stopped crying,” Aydin said. “He was 5 or 6 months old. I had a feeling: Even this baby knows why we’re fighting. I will never forget this.”

One afternoon at the end of March, I received a message from my activist contact in Nusaybin. She had fled the city. There were reports of terrible firefights that day, with tanks blasting buildings into rubble in an effort to recapture the P.K.K. neighborhoods and young rebels struggling to push them back. Omer Aydin, she said, was dead.