Putin's agenda is quite clear. It's about gas, plus US geopolitics.Hopefully most people acknowledge that the Iranian troops and Hezbollah have the same agenda and their agenda is mostly aligned with Assad's agenda. Putin's agenda is different (wants to show Russia as a superpower) but in action he is aligning himself with Assad agenda.
The rebels on the other hand have different agendas from each other and to a fair extent want to kill each other as much as they want to kill Assad. There might be some collaboration on specific issues and I'm sure there are people in all groups putting personal profit ahead of politics.
1. Gas: Russia is trying to get its gas to European markets. They presently supply 40% of Europe's gas, mainly through the Northstream pipeline, in addition to any gas still flowing through Ukraine. The Southstream pipeline project was abandoned when the EU decreed regulations that Russia could not have a monopoly over its use, despite paying for all of it. Putin told the EU to get stuffed and personally cancelled it. He get a deal with Erdogan in Turkey to run a different route under the Black sea to Turkey, and then to be distributed to Southern Europe. Since then, the deal seems to have been scuppered by Turkish actions and Russian retaliation.
The newest and largest gas find in history is shared by Qatar and Iran in the Arabian/Persian Gulf. Qatar would like to sell this gas by running a pipeline directly to Europe, but it has to run through Syria, thus competing directly with Russia. Syria is Russia's friend and Assad denied Qatar's request to run a pipeline through it. Qatar was furious and decided to undermine the Assad regime by funding Al-Qaida/ISIS terrorists in Syria. There was blowback in Iraq, but the ISIS goal is to oust Assad. Qatar has also been funding other rebels, including the leading Jabhat Al-Nusra. When it was clear that Assad's army was in danger of collapse, Russia intervened in Syria, thus attempting to protect its european gas market by propping up Assad militarily. Saudi Arabia also wants to run a pipeline through Syria, and Prince Bandar actually threatened Putin with unleashing Islamic terrorists (what became ISIS that Saudi Arabia initially funded) into the Caucasus republics of Chechnya and Dagestan if Russia didn't push Assad into accepting; Putin told him to go and fuck himself.
2. US geopolitics. It has been the policy of the US for about 30 years now to isolate the Soviet Union/Russia, undermine it, split it up and exploit its resources. This was the master plan of Zbigniew Brzezinski and his Grand chessboard. It was under him that the US first funded Islamic 'Mujaheddin' (terrorists led by Osama bib-Laden) in order to defeat a competing superpower by proxy in Afghanistan. The US will not tolerate countries that do not comply with its objectives, and the US will engage in regime change, either through military action, or through subversion. Since Putin, Russia has become richer, more powerful and is pursuing its own foreign policy that suit its own goals. The US has overthrown the leaders of at least 40 countries since WW2. The US wants to see Russia out of the Middle East, and wants to see the ex-Soviet republics turn against their former masters. Russia is pushing back now, unlike during the Yeltsin era, when Russia was up for sale (and plunder), and so weak militarily that it did not have any foreign policy interests. Since Brzezinsky, there was the Project for a New American Century written by neocons at the American Enterprise Institute, who became the core of Dubya Bush's cabinet, and who decided to get rid of Saddam because he didn't do what he was told. Gen Wesley Clark, who was the Chairman of the US Joint Chiefs of Staff, was shown a policy paper that called for the overthrow of 6 Muslim regimes: Iraq, Lebanon, Syria, Libya, Iran and Afghanistan. That was in 2001 before 9/11..........
This is the big picture.