With the impending withdrawal of American forces, however, the Kurdish forces in the northeast have turned toward the government in Damascus. They are in talks about reaching a deal that could allow Mr. Assad and the Kurds to coexist there as Turkey, which regards the Kurdish fighters on its southern border as a security threat, threatens to roust them from the area.
Mr. Erdogan has called for the United States to help it establish a “safe zone” in northeastern Syria, a 20-mile buffer region east of the Euphrates River that would keep Kurdish forces away from Turkey’s border, and has said Turkey will move ahead with it even without American assistance.
Turkey’s focus on Kurdish militants in northeastern Syria may have inadvertently allowed Hayat Tahrir al-Sham to sweep to dominance, said Ismail Hakki Pekin, a former chief of Turkish military intelligence.
Mr. Pekin said Turkish intelligence officials had siphoned off Turkish-backed rebel fighters in Idlib for operations against Kurdish forces in northeastern Syria. Depleted of manpower, the moderate rebels remaining in Idlib could not resist Hayat Tahrir al-Sham when it attacked.
“Maybe Russia will persuade Turkey to let them enter Idlib,” he said. If so, he said, “they will kill a lot of people and will cause a huge inflow of refugees into Turkey.”
Hayat Tahrir al-Sham’s rise to power could further complicate humanitarian operations in Idlib: Western governments that provide much of the funding for humanitarian aid have become leery of channeling donations to Idlib in recent months over concerns that the money would end up bolstering a terrorist group. Hayat Tahrir al-Sham has also made moves to insert itself into the distribution of aid, though it recently backed down in one key instance, Mr. Heller said.
The United States has designated the group a terrorist organization, and at least two aid groups had to scale back their work in Idlib last year after the United States’ main aid agency put restrictions on grants. Several Western-funded stabilization programs have been suspended.
“Idlib is the catchall for the most vulnerable,” said Rachel Sider, a spokeswoman for the Norwegian Refugee Council, an aid group that operates across Syria. “Aid agencies will again adapt to the context to keep reaching the most vulnerable through the most direct means, including from areas now under government control.”