Michael Koplow at Foreign Affairs:

But the chaos on Turkey’s border with Syria threatens to upend [Turkey's stability]. The rise of the Islamic State of Iraq and al-Sham (ISIS) has threatened Turkey’s internal balance in a number of ways. But the danger does not come from ISIS itself. Although the group has proved its military bona fides during its rampage through Iraq and Syria, it does not present a serious territorial challenge to Turkey, which has a large NATO-backed army, a modern air force, and the resources to hit back at ISIS should it choose. Rather, it is the follow-on effects of ISIS’ march through the region that may herald a return to the bad old days.
During the Israeli-Palestinian conflict, the we (the West) failed to anticipate the impact of Palestinian refugees on Jordan. And when Jordan expelled them, again there was a failure to understand their impact on Lebanon. And when the Iraq War started, there was a failure to understand the impact of refugees on Syria. And now that Syrian refugees are flowing into Turkey (and fighters in the opposite direction), what will be the impact within Turkey?