Iran-Syria Relations and The Arab Spring
This is an old article, but it is well-sourced, well-documented and in my opinion still relevant.
http://www.irantracker.org/foreign-relations/iran-syria-relations-and-arab-spring
Pro-reform and anti-regime street protests have rattled President Bashar al Assads regime in Syria for several months since the Arab Spring sparked unrest across the region. The challenge to Assads rule in Syria has significant regional implications. The Syrian regime is Irans key strategic partner in the Middle East, serving as Tehrans link to terrorist groups Hamas and Hezbollah, both of which threaten Israels security and regional stability. Iranian leaders, who seek to preserve the Assad regime, have publicly downplayed the significance of the Syrian protest movement while reportedly assisting Assad in his violent repression of the internal opposition.
Tehran has cultivated and expanded its relationship with Damascus in recent years.[1] Examples of these ties include reports involving Irans financing of Syrian weapons purchases and the construction of an Islamic Revolutionary Guard Corps (IRGC) base in Damascus.[2] In July 2007, Damascus and Tehran reportedly entered into an agreement under which Iran would supply Syria with Iranian-made weapons in exchange for a Syrian pledge to not enter peace negotiations with Israel.[3] Ties between the two countries also extend to economic cooperation. Syria reported in January 2007 that Iran was the top non-Arab economic investor in the country.[4] More recently, Iranian and Syrian officials have begun negotiations over energy sector projects, including a natural gas pipeline project that would involve Iraq.[5]