It’s a bit complicated. Generally speaking the whole Asad’s regime is backed financially by Iran and militarily by Russians. The reason being, Syria acts as a nearby ally working as a military base for their presence against Israel and Arab nations in the region. Iran used Lebanon for a while, but burnt that card when they were negotiating the nuclear deal (read give up supporting Hamas and Hezbollah and you can keep your nuclear factories). Syria is their last ally in that region. Regarding Russians, it’s just a political game against the EU and the US (NATO). Russians are waiting for a good deal to gain more power at the expense of giving up Syria, which no one is willing to offer at the moment. Same goes for Iran and its issues with Israel, which basically initiated as a propaganda campaign for internal use after the 79 islamic revolution (read Palestinians are Muslim brothers/nation and we want them to have their country back as a Muslim country it used to be). There is no resolution to that either due to failure of Isreal-Palestine negotiations for the past decades. So, it’s become the cluster fuck that you see. It’s all about politics, ideologic differences, and greed for power from both sides at the expense of Syrian lives, which none of the parties apparently care about.