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Russian President Vladimir Putin (R) shakes hands with his Syrian counterpart Bashar al-Assad (L) during a meeting at the Kremlin in Moscow on October 21, 2015. Assad, on his first foreign visit since Syria's war broke out, told his main backer and counterpart Putin in Moscow that Russia's campaign in Syria has helped contain "terrorism". AFP PHOTO / RIA NOVOSTI / KREMLIN POOL / ALEXEY DRUZHININ        (Photo credit should read ALEXEY DRUZHININ/AFP/Getty Images)
Russian President Vladimir Putin (R) shakes hands with his Syrian counterpart Bashar al-Assad (L) during a meeting at the Kremlin in Moscow on October 21, 2015. Assad, on his first foreign visit since Syria's war broke out, told his main backer and counterpart Putin in Moscow that Russia's campaign in Syria has helped contain "terrorism". AFP PHOTO / RIA NOVOSTI / KREMLIN POOL / ALEXEY DRUZHININ        (Photo credit should read ALEXEY DRUZHININ/AFP/Getty Images)

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Understanding Syria's complex battlefield

CNN's Fred Pleitgen looks at the key forces and allies in the Syrian conflict.

Understanding Syria's complex battlefield

CNN's Fred Pleitgen looks at the key forces and allies in the Syrian conflict.