I recently heard, for the umpteenth time, someone from academia smugly denying the validity of the domino theory. That was the argument made during the 1960s by proponents of American military involvement in Vietnam, that, if we allowed a Communist takeover of South Vietnam, other non-Communist governments in Asia would proceed to fall like a line of dominoes.
When we allowed just that, in 1975, much of Asia was spared. But anyone completely dismissing the domino theory has a couple million Cambodian souls to answer to.
Whatever one thinks about that argument, it's clear that, in 1989, the domino theory worked in reverse.
Once the Marxist-Leninist regimes of Poland, Hungary and East Germany fell, what little legitimacy the other such governments had, completely disappeared. Bulgaria and Czechoslovakia fell in line, shortly after the opening of the Berlin Wall.
As of November of 1989, the end of European Communism had come about with remarkably little violence. In fact, the process in Czechoslovakia was so peaceful, it was called the "Velvet Revolution".
Romania was a tougher situation. Nicolae Ceausescu had been one of the most corrupt and hard-line of the dictators. He held out longer than his counterparts in the other Soviet satellite countries. By December of 1989, the situation had grown violent, as Ceausescu tried to use his security apparatus to maintain his power, in the face of a growing popular revolt. His effort ended on Christmas Day, when he and his wife Elena were executed.