The surprise American withdrawal from parts of northern Syria last week reshuffled old alliances and touched off a new stage of the eight-year war. The only certainty is uncertainty — but the answers to these four questions will shape the country’s future.

1. Who will control northeast Syria?

A swath of Syria that had been relatively stable since the defeat in March of the Islamic State’s self-proclaimed caliphate has plunged back into chaos.

As one military force (a small American contingent) abruptly pulled out, two competing ones (Turkey and the Syrian government) pushed in.

Just over a week ago, a roughly triangular slab of northeastern Syria, about a third of the country, was controlled by a Kurdish-led Syrian militia backed by the United States. That militia, the Syrian Democratic Forces, was the main ground force that defeated ISIS in Syria.

But on Oct. 6, President Trump gave Turkey the green light to cross the border and attack the group.

Where Turkish forces and the Syrian government have moved into Kurdish-held areas U.S. forces withdrew on Oct. 7. Qamishli Turkey Kobani Turkey’s proposed buffer zone Ras al Ain Turkish airstrikes M4 Tel Abyad Turkish army AND syrian opposition Suluk M4 Hasaka M4 Manbij Tel Tamer Ain Issa Aleppo U.S. troops deployed to a bridge on Oct. 14. Syrian Army forces entered these towns on Oct. 14. Idlib Raqqa Other opposition Latakia KURDISH Control Deir al-Zour Hama Government Control Homs Mediterranean Sea Palmyra Abu Kamal lebanon Syria Iraq Damascus ISRAEL Sweida Dara’a Jordan 50 MILES U.S. forces withdrew on Oct. 7. Qamishli Turkey Kobani Ras al Ain Turkish airstrikes M4 Tel Abyad Turkey’s proposed buffer zone Turkish army AND syrian opposition Suluk M4 Hasaka M4 Manbij Tel Tamer Ain Issa Aleppo U.S. troops deployed to a bridge on Oct. 14. Syrian Army forces entered these towns on Oct. 14. Idlib Raqqa Other opposition Latakia KURDISH Control Deir al-Zour Hama Government Control Homs Mediterranean Sea Palmyra Abu Kamal lebanon Syria Iraq Damascus Sweida Dara’a Jordan 50 MILES Turkey U.S. forces withdrew on Oct. 7. Qamishli Turkish army AND syrian opposition Kobani Ras al Ain Turkish airstrikes M4 Tel Abyad M4 Suluk Hasaka Turkey’s proposed buffer zone Manbij Tel Tamer Ain Issa Aleppo Syrian Army forces entered these towns on Oct. 14. Raqqa Idlib U.S. troops deployed to a bridge on Oct. 14. Other opposition KURDISH Control Latakia Deir al-Zour Hama Government Control Homs Palmyra Abu Kamal lebanon Syria Iraq Damascus Sweida Dara’a Jordan 50 MILES Government control Kurdish control Other opposition Turkish forces and Syrian opposition U.S. forces withdrew on Oct. 7. Turkey’s proposed buffer zone Qamishli Turkey Kobani Ras al Ain Tel Abyad Turkish airstrikes M4 Suluk Tel Tamer Aleppo Ain Issa Syrian Army forces entered these towns on Oct. 14. Raqqa U.S. troops deployed to a bridge on Oct. 14. Deir al-Zour Homs Palmyra Abu Kamal Syria Iraq Damascus Dara’a Sweida Jordan 50 MILES Government control Kurdish control Other opposition Turkish forces and Syrian opposition U.S. forces withdrew on Oct. 7. Turkey’s proposed buffer zone Qamishli Turkey Kobani Ras al Ain Turkish airstrikes Tel Abyad M4 Suluk Tel Tamer Aleppo Ain Issa U.S. troops deployed to a bridge on Oct. 14. Raqqa Syrian Army forces entered these towns on Oct. 14. Homs Palmyra Abu Kamal Syria Iraq Damascus Dara’a Sweida Jordan 50 MILES Sources: Control areas as of Oct. 14 via Conflict Monitor by IHS Markit; WorldPop

Turkey had long been angry about the American alliance with the Syrian Democratic Forces, known as the S.D.F. The militia’s leadership has ties with a Kurdish group that has fought an insurgency inside Turkey for years; Turkey considers the group to be a terrorist organization.

Turkey wants to establish a buffer zone by driving the S.D.F. out of a strip extending at least 30 kilometers into Syria along the border.

At least 160,000 people have fled the Turkish assault.

And, facing a rout without support from its old allies, the militia’s leaders turned for help to the Syrian government, a foe of both Turkey and the United States. Over the weekend, the Kurds said they had struck a deal for Syrian government forces to re-enter areas they had ceded to Kurdish forces years ago as an uprising swept the country.

The sudden turn of events is creating new risks, as instability and violence increase. And Turkey is seeking to push still deeper into Kurdish territory, using Syrian opposition fighters, mostly Arab and Turkmen, as ground troops. That stokes the potential for ethnic conflict.

Turkey, a NATO member, is competing for territory with the Russian-backed Syrian government, their forces racing to take cities near the border. That raises the prospect of a NATO-Russian conflict, but some experts and observers on the ground believe Turkey and Russia have a deal to carve up the map. They have increasingly worked together on Syria, even as they officially back opposing factions.

At stake is the fate of some four million Syrians living under S.D.F. rule who had found a respite from repression — both from ISIS and from the Syrian government, which has bombed its own cities and sent tens of thousands of people to torture prisons to stay in power.

Recent American statements have called for Turkey to curb its actions, and the United States has sent mixed signals about whether it will try to keep some forces in the area.

But experts said that by allowing the Turkish incursion to begin at all, the United States has effectively ended its military protection of the S.D.F., at least for now. The Turkish incursion makes any continued American presence untenable, they said, by cutting supply routes and undermining locals’ trust in the United States.

2. How will this turn out for the Kurds?

Without American backing, the Kurds are facing an enormous blow to their hopes of retaining a degree of autonomy. They have lost leverage in any future dealmaking with either Turkey or the Syrian government.

And questions remain about precisely what agreement the Kurds reached with the Syrian government. Each side has cast it somewhat differently.

The S.D.F., which has almost never fought the Syrian government or its allies, has suggested that the agreement entails allowing Syrian forces to enter its areas and raise the Syrian flag, to deter Turkey from attacking. It says it will retain its military structure and control of local governing councils.

Where Syria’s major demographic groups live Mostly Kurdish Mostly Arab Turkey Minority groups Manbij Hasaka Aleppo Idlib Raqqa Latakia Deir al-Zour Hama Mostly uninhabited areas Homs Mediterranean Sea Palmyra Abu Kamal lebanon Syria Iraq Damascus ISRAEL Sweida Dara’a Jordan 50 MILES Mostly Kurdish Mostly Arab Turkey Minority groups Manbij Hasaka Aleppo Idlib Raqqa Latakia Deir al-Zour Hama Mostly uninhabited areas Homs Mediterranean Sea Palmyra Abu Kamal lebanon Syria Iraq Damascus Sweida Dara’a Jordan 50 MILES Mostly Kurdish Mostly Arab Minority groups Turkey Manbij Hasaka Aleppo Idlib Raqqa Latakia Deir al-Zour Hama Mostly uninhabited areas Homs Palmyra Abu Kamal lebanon Syria Iraq Damascus Sweida Dara’a Jordan 50 MILES Mostly Kurdish Mostly Arab Minority groups Turkey Manbij Hasaka Aleppo Idlib Raqqa Latakia Deir al-Zour Hama Mostly uninhabited areas Homs Palmyra Syria Damascus Iraq Dara’a Sweida 50 MILES Jordan Mostly Kurdish Minority groups Mostly Arab Turkey Manbij Hasaka Aleppo Idlib Raqqa Latakia Deir al-Zour Hama Mostly uninhabited areas Homs Palmyra Syria Damascus Iraq Dara’a Sweida 50 MILES Jordan Note: The broad demographic categories shown here cover a diversity of ethnic and religious groups. The predominantly Kurdish and predominantly Arab areas are made up of mostly Sunni Muslims. Minority groups include Christians, Druze, Shiites, Alawites and others. | Source: Dr. Michael Izady for the Atlas of the Islamic World and Vicinity (New York, Columbia University, Gulf 2000 Project: 2006-present).

But the Syrian government has said it will require the S.D.F. to disband and put its fighters into Russian-run fighting formations like the Fifth Division, which has absorbed surrendering Syrian rebels from other parts of the country. That would be more in keeping with precedent.

Damascus did once effectively cede the northeast to Kurdish forces early in the Syrian uprising as it focused on quelling Arab opposition fighters in the country’s more populous west. And until now, it has even kept offices in two Kurdish cities and an airport.

Kurdish leaders, never fully trusting the United States, have always kept channels open to Moscow and Damascus.

But Damascus has a long history of repressing Kurds, and President Bashar al-Assad’s government is not known for making deals: In areas it has retaken, it has insisted on total surrender, with no concessions. And it has punished those who defied it, conscripting and even disappearing those who sought autonomy.

Some believe the Kurds may fare better. But in majority-Arab areas, the mood is fearful. That opens a door for resurgence of armed opposition, including extremists.

3. How are civilians being affected?

Some 160,000 civilians — many of whom have already been displaced repeatedly — have already fled the border zone that Turkey is attacking. Roads out of major cities are clogged, and families packed into cars are telling reporters that they have no idea where to go. Routes to Turkey are blocked.

Some are trying to flee to the Kurdish region of Iraq, others to Syrian Democratic Forces territory further south, where militia officials say they are relocating some refugee camps.

But both of those regions are exhausted and destroyed after years of battling the Islamic State, and have few resources to offer refugees. To make matters worse, the international relief group Mercy Corps is pulling out of northeastern Syria because it can no longer reach people in need. In a statement, it called that “a nightmare scenario.”

Civilians fleeing the Syrian town of Tel Abyad amid a military operation by Turkey against Kurdish fighters in northeast Syria.

North Press Agency via Reuters

Civilian injuries have already been reported, some attributed to Turkish shelling. Civilians have also been killed in Turkey in cross-border shelling by S.D.F. fighters.

Another threat could emerge if Kurds in the S.D.F. decide to fade into the population and carry out an insurgent campaign against the Turks.

Turkey has tried to sell its border zone to some Syrians as a Syrian opposition protectorate that could foster some local autonomy and restore the homes of Arabs who have been displaced by Kurds. But the Syrian rebel groups that work with Turkey have lost credibility, with some dismissing them as “Turkish mercenaries.”

Syrian Arab and Kurdish civilians arriving in Hassakeh last week after fleeing Turkish strikes in northeast Syria. Delil Souleiman/Agence France-Presse — Getty Images

Turkey has also said that it wants to push many of the 3.6 million Syrian refugees living in Turkey over the border and into the buffer zone it is trying to make in Syria, a move that would violate international law. The largely rural area has no capacity to absorb that many refugees, who come from all over Syria.

4. Will ISIS come back?

The chaos of the last week has opened the door to two possible threats from ISIS: the escape of former fighters detained when the group was defeated, and the reactivation of sleeper cells the militants are believed to have put in place.

Thousands of suspected militants were being held in detention facilities throughout S.D.F. territory. They include at least 2,000 foreign citizens whose home countries have so far refused to take them back.

Some of the prisons are located within the 30-kilometer strip that Turkey has vowed to seize. So are some of the camps holding tens of thousands of people from areas once held by ISIS, including many women and children.

Where Kurdish fighters were guarding ISIS detainees Qamishli Turkey Kobani Ras al Ain Tel Abyad Manbij Hasaka ISIS detention facilities Aleppo Ain Issa Idlib Raqqa ISIS members’ families escaped from detention. Latakia Deir al-Zour Hama Homs Palmyra Abu Kamal lebanon Syria Iraq Damascus Government control Kurdish control Other opposition Dara’a Turkish forces and Syrian opposition Sweida Jordan 50 MILES Detention facilities holding ISIS members Government control Kurdish control Other opposition Turkish forces and Syrian opposition Qamishli Turkey Kobani Tel Abyad Ras al Ain Ain Issa Aleppo Raqqa Idlib ISIS members’ families escaped from detention. Latakia Deir al-Zour Homs Palmyra Syria Iraq Damascus Dara’a 50 MILES Jordan Detention facilities holding ISIS members Government control Kurdish control Other opposition Turkish forces and Syrian opposition Turkey Qamishli Kobani Tel Abyad Ras al Ain Ain Issa Aleppo Raqqa Idlib ISIS members’ families escaped from detention. Latakia Deir al-Zour Homs Palmyra Syria Iraq Damascus Dara’a 50 MILES Jordan Sources: Control areas as of Oct. 14 via Conflict Monitor by IHS Markit; locations of detention facilities via Institute for the Study of War

Already, there have been reports of escapes, and American forces were unable to extract dozens of high-value prisoners ahead of the fighting. One prison, in the border city of Qamishli, was hit by Turkish mortars on Friday, and five ISIS suspects fled in the aftermath.

S.D.F. guards have been trying to hold their positions at the prisons, but it is unclear how long they can remain. The main prison site, at Al Hol, lies outside the border strip, farther southeast toward the Iraqi border.

There is also fear that ISIS sleeper cells throughout the area could take advantage of the turmoil and reactivate; the group has already claimed responsibility for one suicide bombing in Qamishli since the Turkish operation began.

Many Arabs fear the return of the ISIS militants. But the threat of a return of Syrian government forces — which could mean torture or conscription, especially for young people in the opposition — could tempt some to support any alternative, even extremists.