The convoluted, twisted strands of who's fighting who, who's supporting who, and who's trying to stay relevant in Syria (and with whom) are almost nightmarish in their complexity. Strategy Page does its best to describe the mess.
Iran, Russia, Turkey, the United States and Israel are all present in Syria along with the Assad government and a considerable number of Syrian rebel groups who are still not united. Everyone has different goals and a different (often constantly shifting) set of allies. Keeping track of who is doing what to whom and why (and for how long) has become increasingly difficult. A current summary of allies, foes, frenemies and chimeras goes like this;
Israel wants to keep Iran out of Syria and Lebanon and avoid a war with Iran. For this Israel has the support of the U.S., Russia and most Gulf Arab states. None of these supporters is willing to provide any military assistance, at least not until Iran actually attacks Israel.
Russia wants to get Turkey out of NATO, to keep the Iranians from starting a war with Israel and make the Americans look bad. At the same time Russia needs to do this on the cheap and make Russia look good, especially to Russians back home. That is proving difficult as most Russians were not enthusiastic about the Syrian operation in the first place and popular support has been declining. Israel sees Russia as being of limited use because of the Russian strategy. Moreover Russia is not as militarily powerful as it pretends to be. Privately the Russians agree with Israel on that and appreciate any help the Israelis can provide in this area.
Turkey wants to create a security zone on the Syrian side of the border that has no Kurds or Islamic terrorists in it. Turkey also wants to show the Sunni Moslem world that it can handle Iranian aggression (without going to war with Iran) and keep the Israelis out of Lebanon and Syria. Turkey is willing to play diplomatic games with Russia and Iran to achieve these goals as well as send troops into Syria to fight, and get killed. Turkey also likes to play (or pretend) tough with Israel. Turkish military experts know better but many Turkish politicians are clueless and that is very dangerous in this part of the world.
The U.S. wants to ensure that ISIL (Islamic State in Iraq and the Levant) does not reestablish itself in Syria. To help with that the American are doing what they can to help the local Kurds maintain autonomy in northeast Syria (east of the Euphrates) where the Kurds have always been dominant. Secondary objectives are keeping Iran, Turley and Russia out of Syria. The U.S. and Israel are allies and any attack on Israel will trigger American intervention on the Israeli side. But short of that the Americans are quite blunt about stating that their troops are in Syria to deal with terrorists, not Iranian preparations for an attack on Israel. The Americans will not stop Kurds in the northeast from going to the aid of Kurds defending Afrin. The Turks asked the Americans to stop the Kurds and the U.S. refused.
There's more at the link.
The same article has some interesting insights into the internal situation in Israel, Iran and other Middle Eastern nations. Recommended reading.