I don't know what the truth is.
The Guardian is generally critical of US claims about the killing of civilians, and has documented the deaths of civilians from eye witnesses and medical experts. I don't recall them using the word "massacre" in recent reporting, and I searched some of their articles from the last two weeks. Perhaps it was an editorial that you recall, or a wire story in which a person was quoted describing the violence in Fallujah as a massacre.
At this time the only claim of "massacre" in Fallujah that has been objectively examined in any detail is the massacre of protesters last year. (See the Human Rights Watch report,
Violent Response: The U.S. Army in Al-Falluja.
What is a massacre? According to Webster's (via dict.org), as a verb it means:
"To kill in considerable numbers where much resistance can not be made; to kill with indiscriminate violence, without necessity, and contrary to the usages of nations; to butcher; to slaughter; -- limited to the killing of human beings."
As a noun:
"The killing of a considerable number of human beings under circumstances of atrocity or cruelty, or contrary to the usages of civilized people."
Large numbers of civilians were killed in Fallujah. That is a fact. Has the killing been indiscriminate? There is evidence that strongly suggests that. It must be looked at.
Uncivilized? That kind of talk pains me. Even in words, the barbarism eminating from the New York Times, rivals the militant xenophobia of, I don't know,
Islamic Awakening. So I plead agnosticism on the question of whether civilization exists, whether it is, in the final analysis, distinguishable from barbarous cruelty. I suppose there's always the law, but international law has seen brighter days, and justice brighter still.