Things are getting more complicated and confusing by the day in Syria. StrategyPage offers a detailed examination of the complexity of the situation. Here are a few excerpts.
While on paper ISIL (Islamic State in Iraq and the Levant) seems doomed in Syria this is not the case. No, what is keeping ISIL alive are growing disagreements among the many nations and factions mobilized to crush ISIL in Syria. This is a sad situation but also so typical of the region.
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In Syria most of the attacking around Raqqa has been by the SDF (Syrian Democratic Forces) rebels, an organization composed of Syrian Kurds and their Moslem and Christian allies. Some 70 percent of SDF forces advancing on Raqqa are Arab, the rest are from various Kurd factions ... SDF began another (often delayed) push towards Raqqa on the 13th and since then have killed over 300 ISIL gunmen and lost about three dozen of their own. This SDF effort is supported by American ground troops, artillery and airstrikes.
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Now the Turks are deliberately attacking Kurdish forces including SDF and its non-Kurd allies.
That is mainly because Turkey is trying to prevent the Syrian Kurds from establishing an autonomous region in northern Syria. The Turks are the only member of the anti-ISIL coalition that wants to keep the Kurds out of the final offensive to crush ISIL in Syria. The Turks are also opposed to the growing Iranian presence in Syria and Iranian plans to make that presence (and control of the Syrian government) permanent. Israel also opposes the Iranian presence but is neutral about the Kurds and has the support of Russia and the United States for that. Russia and the United States are trying to prevent the offensive against ISIL from being disrupted because of growing hostility between the Turks and the SDF but are not having much success.
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While Russia is officially in Syria to defeat ISIL and keep the Assads in power they (semi-officially) also want to maintain good relations with Israel and Sunni Arab countries while doing it. This annoys Iran. Israel has made it clear that there can never be peace in Syria if Iran tries to establish a permanent presence there. The Iranians say they are in Syria to stay and the Russians (so far) have said they oppose that. Iran wants to stay in Syria as part of its decades old effort to destroy Israel and a centuries old effort to make the Shia form of Islam dominant in the Islamic world (that is over 80 percent non-Shia). Meanwhile Israel says it can live with the Assads as long as Iran is not maintaining a military presence in Syria. Many Turks agree with Israel on that point and newly elected U.S. government has come out strongly against any permanent Iranian presence in Syria. The Americans still want the Assads gone but despite that the U.S., Israel and Turkey agree on some key goals.
The Russian intervention appears to be permanent (as far as the Russians are concerned) and the Assads are OK with that. Turkey and Iran are not so sure and Iran is openly opposed to Turkish troops being in Syria at all. At the same time Iran is demanding the right to establish a naval base in Syria. This is not a new idea. In 2011 Iran pledged to pay for the construction of a naval base on the Syrian Mediterranean coast. That proposal was put aside as the rebellion against the Assads grew but now Iran wants some payback for playing a key role in maintaining the Assads in power. All these overlapping and often contradictory goals and alliances may seem odd to an outsider but this is the Middle East, where such complex arrangements are the old normal.
There's much more at the link. Recommended reading.
This is the tangled web into which US forces have been sent, albeit in limited numbers at present, but risking escalation and possible reinforcement in the not too distant future. It also involves Israel, the staunchest US ally in the region, and one that might be drawn into a more active role in the conflict in response to Iranian activities and machinations. (A good example is the Israeli air strike on a Hezbollah weapons storage facility outside Damascus last week. If Iran attempts to increase weapons deliveries to Hezbollah, expect such strikes - using US-sourced aircraft and weapons - to become larger and more frequent.)
The situation in Syria is probably the second most difficult foreign policy and military dilemma the USA faces at present, second only to North Korea. (Given Iran's relatively close military relationship with North Korea - it's dependent on NK technology for many of its missiles - the two situations may even be entwined, with Iran planning action in Syria to divert US attention and resources away from NK.) We'll do well to keep a close watch on what's happening there.