While flawed in practice, I think the so-called "domino theory" had some valid roots in the so-called Paradox of Tolerance.
It's hard to dispute one historical aspect of communism in particular, namely that it cannot tolerate competing ideologies. The only way to implement communism is through the use of force; it's not something free people will adopt voluntarily. I may think of myself as a tolerant person, but if you have to stick a gun in my back to make me practice your ideology -- which you do -- why should I be required to tolerate that?
It can be argued that it's a moral responsibility to stop the spread of such a pathological system of thought by any means necessary, just as if we were fighting an infectious disease. I won't go that far personally, but others in America did, and the wars in Korea and Viet Nam were the result. Not so much an apology as an (admittedly oversimplified) explanation.