Russia and Syria: Arms and Alliances

The story referenced below gives a good overview of the relationship between Russia and Syria.  It is especially interesting that the ties that have made Russia and Syria "allies" are commercial and center on arms.  Russia, by supporting Syria with weapons, is able to maintain influence in the region.  This is an interesting contrast to the deteriorating arms relationship with Iran.

As international pressure on President Bashar al-Assad of Syria grows, Russia has maintained a perplexing timidity towards developments. Moscow has steadfastly refused to stake out an unambiguous position on events in Syria, a diplomatic paralysis that may end up proving more costly to Moscow in the long run. 

Some analysts say the Kremlin's careful stance is a result of its unwillingness to lose its only real Middle Eastern ally and a desire to avoid a confrontation with the West. 

"Syria remains Russia's only ally in the Middle East," Vladimir Karyakin from the state Institute for Strategic Research said. "We abandoned the rest either during perestroika or during the recent Arab revolutions. We even betrayed some - like Libya or Egypt, for example."
Russia has been a major arms supplier to Syria since the Soviet era and political cooperation with Damascus has often been far more valuable to Moscow than money. In 2005, Russia wrote off more than 70 percent of Syria's $13-billion debt, much of which was the result of Soviet-era arms deliveries. 

Although financial interests now play a more important role in defining Moscow's approach to Syria than during the Cold War, political concerns still remain the cornerstone, analysts say. Since the early 1970s, the country has hosted Russia's only naval supply and maintenance base outside the former Soviet Union in its Mediterranean port of Tartus.
"If we lose such an ally, we will lose our foothold in the Middle East," Karyakin warned.

Conflicting interests paralyze Russian diplomacy on Syria: analysts | Features & Opinion | RIA Novosti:

The following graphs show the growing importance of Russian arms imports to Syria.  I put together the graphs using data from SIPRI.