They length of time and number of dead Americans

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There’s also an angle that the realities of Vietnam were televised, and the American people saw the realities of war in fidelity that the overwhelming majority had never seen before. As far as I’m aware, that degree of journalistic access essentially hasn’t occurred in future wars, because it had such a detrimental effect on public support. So much of the cruelty of Vietnam was documented in excruciating detail, and the US was responsible for the overwhelming majority of it.

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