woodsThe domino theory assumed that Communism was a monolith. It did not allow for intra-Communist squabbles, of which there have been many: the Soviet Union and China, the Soviet Union and Tito’s Yugoslavia, Vietnam and Cambodia, etc.
Secondly, what is it about Communism that should make us think it was destined to take over the world? Every country the Soviet Union acquired wound up being an additional economic drain. The Soviet Union was a basket case economically in any event. This backward, primitive thing was going to take over the world? I don’t see how.
Both the Left and the neoconservative Right promoted preposterous estimates of Soviet strength — the Left in order to show that central planning really did work, and the neocons so they could justify the expansion of the military/interventionist state they favored. Both were wrong.
Finally, I recommend looking at the references linked with the lectures on the Cold War in U.S. History Since 1877.