The US War in Vietnam, it seems to me, was misguided and to fight it we (the US) supported many un-democratic elements and engaged in very questionable actions -- actions that in many ways directly contradicted not only our stated goals in the country, but our stated values.
Having said this, context -- it seems to me -- is also important. Yes, the US supported "reactionary" forces, yet at the same time the Russians were crushing democratic and freedom movements throughout Eastern Europe -- Poland, E.Germany and Hungary comes to mind. Further, Mao was concentrating his own governance of China and laying the groundwork for famine and disaster that ended up in the deaths of millions.
My point is not to excuse US failings, but to suggest that there were more horrible things going on in the world. Now, you can look at the Vietnamese situation and argue that the North was a liberation movement and a reunification effort. All likely true. However, these were not democrats and they were not -- and as importantly did not -- create democratic institutions. It is hard to argue that the re-education facilities etc. established by the victors were somehow better, more humane or gentler to victims than the various practices of the South regime or its US patrons.
Hundreds of thousands if not millions fled "democratic" united Vietnam both because of the regime, as well as because of their work with/for the US and the South regime.
Today, Vietnam is a one party state where dissent is persecuted, where party hacks dominate the system, the beauracracy stifles both creativity and innovation, and there is little hope of real reform. As importantly, this was always the likely outcome of the North's victory and reunification effort -- given its underlying allegiance to Leninist and Stalinist methodology and thinking.
Now, while there was little that was good or admirable about the South or US involvement in that Civil war, it strikes me that there is nothing particularly good or admirable about the current situation in Vietnam -- save that it is "unified". Perhaps that is enough. Perhaps the US effort that stifled "unification" is in itself sufficiently evil to merit condemnation.
In any event, Vietnam, Haiti, Guatemala, etc. can all be used to demonstrate the US's ham-handed and often misguided policies.
I only note -- and am not sure how to place it in context -- that at the time when US supported governments and actions were resulting in the torture, death and repression of hundreds of thousands, actions taken by the USSR, China, etc. was resulting in repression and deaths of millions. This is not to excuse misguided policies of the US and its allies, nor is it intended to suggest that many questionable interests directed US policy. Rather it is to suggest that as bad as US policy could be, there was and is real evil out there in the form of dictatorships and systems that use terror not merely as an instrument of control, but at basic state policy -- Saddam, not the least.
Just some, likely misguided if not erroneous observations --
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