Mark N. Katz is a professor of government and politics at George Mason University near Washington, D.C. He is the author of the upcoming book "Leaving without Losing: The War on Terror after Iraq and Afghanistan."

(CNN) -- Many observers point to Moscow's close ties with Damascus going back to the 1950s as a reason for Russia now acting to defend the al-Assad regime in Syria against its many internal and external opponents.

What has apparently been forgotten is that Moscow's ties with Syria have been plagued by tensions and disagreements throughout this entire period.

Underpinning the Moscow-Damascus relationship for over half a century now has been a common antipathy toward America, Israel, and the moderate Arab states. But they have also differed on many issues.

When Soviet-Syrian relations first became close during the mid-1950s, Moscow seemed to hope the then-powerful Syrian Communist Party might at least share power with the virulently anti-Israeli and anti-Western Baath Party. But the Syrian Baathists feared the communists and agreed to the 1958 merger of their country with Egypt and even accepted the leadership of the latter's ruler, Nasser, partly in order to get his help in suppressing the communists....