That sentence is a cognitive litmus test. Some people's immediate reaction is, "That can't be right," because the United States couldn't do that. Or because crimes on that scale don't still happen. Or because they do happen, but only in horrible places that the United States hasn't rescued.
One million is a "Grandpa, what did you do to stop it?" number. It's a number that undeniably puts the American state among history's villains. Those who are not willing or able to accept this are physically unable to retain the fact that over a million Iraqis are dead. Their brains expel it like a foreign germ.
Noam Chomsky once wrote that the "sign of a truly totalitarian culture is that important truths simply lack cognitive meaning and are interpretable only at the level of 'Fuck You,' so they can then elicit a perfectly predictable torrent of abuse in response."