After several recent high-level political and military meetings between Turkey and the United States, Turkey appears to be changing its approach toward the Islamic State. Previously, Ankara had avoided direct participation in the fight against Syrian President Bashar al Assad and against the Islamic State and had refused to host any substantial U.S.-led coalition airpower. Now, however, Turkey has moved considerable forces south, has closed off the final sections of its border adjacent to Islamic State positions in Syria and has opened Incirlik air base — and possibly other air bases for primary or emergency use — to U.S.-led coalition aircraft. Moreover, Turkey has begun carrying out its own airstrikes against select Islamic State positions in Syria.
Increased Turkish involvement will have a significant effect on the war in Syria: First and foremost, the Turkish military moving to secure the border sections abutting Islamic State positions in Syria should seriously degrade a vital logistical line for the Islamic State. Supply lines through Turkey allowed the Islamic State to exchange captured resources — such as oil and historic artifacts — for weapons and equipment as well as money to pay its fighters. The supply lines also kept a steady stream of foreign fighters coming in to replenish the Islamic State's ranks, which allowed it to sustain a high attrition rate. Cutting off the supply line through Turkey will hit the Islamic State hard on all fronts, especially as the effects of the cutoff accumulate over time.
The opening of Turkish air bases makes the air campaign against Islamic State positions in Syria massively more efficient. The previous distances involved for U.S.-led coalition aircraft from various locations in the lower Arabian Peninsula ensured mid-air refueling and longer flight times. Greatly reducing the distance from takeoff to targets in northern Syria gives the coalition more options.
Taking military action against the Islamic State should eventually bring relief for the northern Syrian rebels fighting both the Islamic State and Syrian government forces. This brings U.S. and Turkish interests into alignment, and Ankara likely hopes that the degradation of the Islamic State will lead to a coherent rebel force that can focus solely on Syrian government forces. There is a cost to Ankara's policy change, however: the Islamic State will undoubtedly strike back.