The emotional currents surrounding Vietnam still echo strongly in US politics and the media, which is why it gets hit so often that it's become a cliche. Vietnam is also overwhelmingly obvious evidence of the unimaginable idiocy of allowing politicians to run a war.
Plus it's the clearest defeat of the US on the battlefield in recent memory.
Don't forget either that no few of the politicians who run our country now are related to or indebted to the politicians of that generation (Some are, or were recently, the exact same politicians in fact).
This is a case where the impact of the event far outweighs the realistic ability to compare it to Iraq.

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