I've been watching Turkish intervention in the Syrian civil war with considerable interest. In many ways, Turkey is trapped there. It must intervene to prevent its own Kurdish rebel minority growing stronger, through the latter's alliance with the Kurdish militia in Syria and Iraq; yet, by attacking Turkish Kurds in Syria, it exposes itself to attack from all the other militias and terrorist groups in the area. It also must deal with significant Russian and Iranian intervention in Syria, which is often at odds with what Turkey wants to achieve - and causes further complications within Turkey itself.
Austin Bay wrote a very interesting article recently about the situation in Syria. Here are some extracts bearing on Turkey's involvement.
While the Turks back a peace deal with the Assads they have different goals in Syria. Since late August Turkish forces have taken control of over 2,000 square kilometers of Syrian territory along the Turkish border. The Turks expect to complete their ground operation in Syria by mid-2017. So far about 30 of the thousand or so Turkish troops in Syria have died in operations that have left over a thousand ISIL men dead or captured. While the official goal of the Turks is to join in the international effort to destroy ISIL they are mainly in Syria to limit the presence of Syrian Kurdish rebels bear the Turkish border. Thus the Turks believed their forces have killed about 300 Kurdish rebels in Syria, most of the, members of the Turkish PKK or the similar Syrian YPG. Currently the main Turkish effort is against the ISLI held town of al Bab, which is east of Aleppo and near the Turkish border. Turkey indicates that they might join the advance on Raqqa after al Bab is taken, but only if the Americans get the Kurds to withdraw from the force now advancing on Raqqa. ISIL is taking a beating in al Bab and are expected to lose control of the town by the end of the month.
Syrian Kurds have kept the border areas they control in northwestern Syria free of ISIL and other Islamic terrorist activity but Turkey and Iran remain largely hostile to some of the Kurdish militias in Syria. That’s because the most active Kurdish rebels have belonged to the PYD (a Syrian Kurd separatist group allied with Turkish Kurdish PKK separatists) and their military forces (the YPG). There are also some Iranian Kurds who came to Syria and joined the YPG and other Syrian Kurd rebel groups. The Turks believe that the new American government, that takes over on January 20th might be more flexible.
Since Turkish troops entered Syria in August the Turks have carried out some key tasks that get little media attention. The most obvious of these is increased border security with Syria and Iraq. Since August Turkey has built 89 new military outposts along the Syrian and Iraqi border. Most of those new outposts (and all of the 157 new ones in 2017) were along the 900 kilometers long Syrian frontier. At the same time the Turks are building a concrete wall along 91 percent of the Syrian border and that is about 40 percent done. Since August Turkish forces have seized control of all the roads crossing the border and instated strict border controls that have temporarily put Turkish smugglers out of business, at least those not willing to work closely with the Turkish troops ... By early 2016 the Turks agreed that ISIL must be shut down whatever the cost. The Turkish efforts closed the last useful supply line for ISIL. All the other Syrian borders (with Lebanon, Israel, Jordan and Iraq) are now controlled by governments who are extremely hostile to ISIL. This denies ISIL a way to get new recruits in and people (like families of senior ISIL members and members being sent abroad to help with recruiting, fund raising and planning overseas attacks) out.
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[The Turkish government] has been dominated since 2000 by Islamic parties that depend on Turkish Sunnis (80 percent of the population) to stay in power. Turks have long seen themselves as foes of Shia Iran so the atrocities of Iran backed Shia militias in Syria are seen as another attack on Turkey. This Sunni Turkish nationalism also played a role in the recent assassination of the Russian ambassador by an off-duty Sunni policeman protesting the suffering of Syrian Sunnis in Aleppo. To further inflame Turkish popular anger Russia and Iran recently revealed that once the civil war is over and the Assads are back in charge Iran will be allowed to share with Russia the Syrian naval base at Tartus.
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The Turks are, on paper, the strongest military force in [eastern Syria]. But all Syrians, both the Assad government and the rebels oppose the Turkish intervention. The Turks are mainly doing this because of domestic politics in Turkey. The Kurdish separatists in Turkey (the PKK) are again openly fighting the government and often use bases in Syria. While the Kurds of northern Iraq will cooperate with the Turks in controlling the PKK, some of the Syrian Kurds (the YPG) have worked closely with the PKK before and the Turks do not trust them to behave like the Iraqi Kurds. Meanwhile Turkey is willing to work with Kurdish militias not associated with the YPG.
There's more at the link. Intricate, involved, but very interesting reading from a geostrategic point of view.