Syria and Iran are Russian allies. They had a plan to develop a pipeline from Iran to the Syrian Mediterranean coast to sell oil to Europe. At the time, Iraq had a Shiite Govt and was also allowing the project to go through so the pipeline would've been crossing along Northern Iraq.
Russia had huge investments in the project.
This is basically what the Syrian war is about and is why Russia has a vested interest.
The war is because at the same time, Saudis also had a plan to build a pipeline from SA up through Syria and to the Mediterranean coast.
Whoever wins the war, wins the pipeline.