In a major policy shift, Turkey has agreed to allow the United States to use a key base to launch airstrikes against the Islamic State group and agreed on the outlines of a plan to rout the extremists from a stretch of Syrian territory along the Turkish border.

Here's a look at where things stand:

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WHAT'S AT STAKE

A 110-kilometer (68-mile) stretch along the Turkish border in Syria is under Islamic State control. Turkey and the U.S. agreed to push the group from there, replacing its militants with relatively more-moderate Syrian insurgents and eventually establishing a safe zone where tens of thousands of displaced Syrians could live. The U.S. says the plan does not include the imposition of a no-fly zone, nor Turkish or U.S. troops on the ground. But Turkish Prime Minister Ahmet Davutoglu said air cover will be provided. The deal further embroils Turkey, a key NATO ally, in Syria's civil war, and also catapults it into a front-line position in the global war against the Islamic State group.

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WHY DOES IT MATTER

Turkey's involvement could have far-reaching consequences. An Islamic State-free zone along the Syria-Turkish border — through which the group has smuggled foreign fighters, oil and other goods for the past year — would strangle the militants in Syria, weakening the group. Use of Turkey's Incirlik base would allow U.S. warplanes closer access to Islamic State targets in northern Syria. But a Turkish ground campaign sets up a potential conflict with U.S.-backed Kurdish forces in the area, possibly emboldening other Turkey-backed hard-line Islamic groups.

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WHAT DOES TURKEY WANT

The Islamic State group is a significant threat to Turkey, particularly after a deadly suicide bombing in the country's south that authorities blamed on the group earlier this month. But Turkey's more primary concern is to prevent the establishment of a Kurdish state along its southern border and to topple Syrian President Bashar Assad's government. Syrian Kurds are concerned that Ankara likely is trying to limit advances by Syrian Kurdish forces and trying to steer Washington away from the YPG, the main Kurdish fighting force in Syria. Turkey called a meeting of its NATO allies Tuesday to discuss threats to its security and its airstrikes.

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WHAT AN ISLAMIC STATE-FREE 'SAFE ZONE' MIGHT LOOK LIKE

A Turkish official has said Turkey and the U.S. are discussing "the formation of a de-facto safe zone" which would facilitate the return of refugees, adding that Turkey was prepared to provide assistance including "air support." The zone likely would stretch between the towns of Azaz to the west and Jarablous to the east, and it probably would reach as far south as al-Bab. Close to 2 million Syrian refugees have fled to Turkey. Many may be encouraged to return to Syria under U.S. and Turkish protection. However, in the absence of a no-fly zone to neutralize Assad's warplanes, it is not clear how the possible buffer zone could be considered a safe haven.

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HOW DOES THIS AFFECT THE WIDER CIVIL WAR IN SYRIA

The new Turkish measures potentially could weaken the Islamic State group in Syria and strengthen other insurgents fighting it and the Syrian government. A safe zone north of Aleppo may open the way for rebel groups backed by Turkey to capture Syria's largest city and former commercial center, which has been carved up between government and rebel-held areas since mid-2012.

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Follow Zeina Karam on Twitter at www.twitter.com/zkaram .