Reffering to a report from the Institute for the Study of War, Iran has supported Assad for a long time. The Iranian security and intelligence services are advising and assisting the Syrian military. Iran has been providing essential military supplies to Assad, primarily by air and Lebanese Hezbollah began to take on a more direct combat role in Syria as the Assad regime began losing control over Syrian territory in 2012. The Syrian conflict has already constrained Iranâs influence in the Levant, and the fall of the Assad regime would further reduce Tehranâs ability to project power wich also would lead to reduce Chinas and Russias influence in the whole region.

I know many of you thinking that this institute isnt an reliable Source but in this case, if you look at it from the military and geopolitical point of view, it seems logical.

You can read the whole Report about it here:
www.understandingwar.org/report/iranian-strategy-syria

Also here is an Dutch News Report About Iranian Advisors in Syria wich clearly shows Iranians are involved with boots on the Ground:
https://www.youtube.com/watch?v=z2J02DutU2c

First Reports saying this video shows members of the Iranian Basij Paramilitary Unit in a heavy Firefight in open Terrain against Syrian Rebells. The Basij is a paramilitary volunteer militia established in 1979 by order of the Islamic Revolution's leader Ayatollah Khomeini. The original organization comprised the civilian volunteers whom the Ayatollah Khomeini urged to fight in the Iran-Iraq war. The force consists of young Iranians who have volunteered, often in exchange for official benefits. Now you can clearly see them fighting in Syria alongside the Syrian Army. You can hear clearly speaking Farsi the Persian Language. Ironically Assad and his allies claiming to defend Syria from invading terrorists groups wanting to set up the Sharia Law, a Law practiced by The Islamic Republic of Iran, in Syria.