Syrians fleeing the war pass through border fences to enter Turkish territory illegally, near the Turkish border crossing at Akcakale in Sanliurfa province on June 14, 2015. AFP PHOTO / BULENT KILICBULENT KILIC/AFP/Getty Images

The scope of the war against the Islamic State is expanding. As outlined by the Washington Post on Sunday night, Turkey and the United States have reached an agreement that calls for a de facto “safe zone” along the Turkey-Syria border after militants are driven out of 40-mile wide, 68-mile-long area west of the Euphrates River.

The agreement appears to be a compromise between implementing a full no-fly zone that would protect Syrian refugees from airstrikes by Syrian President Bashar al-Assad’s forces, as Turkey has demanded, and making a greater commitment that could pull the United States into war with the Syrian government. It puts rebels with the Free Syrian Army and other groups in a leading role on the ground to provide security against militants, without explicitly stating that there will be repercussions for Assad’s regime if it launches airstrikes in the zone.

[Booby-trapped corpses and other horrors in Syrian town after combat]

The agreement raises many questions. For one, there is deep-seeded mistrust and a history of bloodshed between the Turks and the Kurds, who exert control over sections of northern Syria and Iraq. The United States has launched hundreds of airstrikes to help the Kurds in their fight against the Islamic State over the last year, but Turkey is against any effort that could lead to the Kurds setting up their own state in northern Iraq or Syria.

“I am saying this to the whole world: We will never allow the establishment of a state on our southern border in the north of Syria,” Turkish President Recep Tayyip Erdogan said last month. “We will continue our fight in that respect whatever the cost may be.”

The situation also is complicated by the involvement of the Kurdistan Workers’ Party (PKK), a Marxist-inspired group that is considered a terrorist organization by both Turkey and the United States. It has fought Islamic State militants repeatedly, but in Turkey has long used guerrilla tactics, including assassinations and bombings, while agitating for a Kurdistan that is independent from other governments.

The PKK called for a ceasefire in 2013 with Turkey, but took credit last week for the killing of two Turkish police officers that it said had collaborated with the Islamic State to carry out a suicide bombing. The Turkish government began bombing PKK camps in addition to Islamic State targets afterward. The PKK remains allies with the Syria’s Democratic Union Party (PYD), which regularly fights the Islamic State, controls territory in northern Syria and benefits from U.S.-led airstrikes.

Brett McGurk, the deputy special presidential envoy to the coalition countering the Islamic State, responded in this fashion afterward:

It also isn’t clear yet how the Free Syrian Army rebels who would protect the safe zone will operate, or how they will be protected from the air. A U.S. official with knowledge of the program through which the U.S. Defense Department is training the rebels told The Post last week that they have the equipment and training to call in airstrikes, but have not been authorized to do so.

If they are, vetted rebels would radio their location and their enemy’s to teams in the region to coordinate with U.S. and coalition aircraft. The information would then have to be corroborated using other intelligence methods — an effort to avoid mistakes, but one that will add to the amount of time needed to launch airstrikes.

It also isn’t clear yet how the Turks, the Kurds or the United States will convince refugees to gather in the de facto safe zone. There are more than 4 million Syrian war refugees, spread out in Turkey, Lebanon, Jordan and other countries.

Those looking to find refuge in the planned safe area will face struggles to get there and are likely to have questions about how it will be protected. It will require a significant ground force to keep the peace. As the Daily Beast pointed out last month, local media reports in Turkey suggest the deployment of thousands of Turkish soldiers to Syria is imminent. That mission is seemingly open-ended.