I don't doubt for a moment that American and other Western soldiers were trying hard to avoid civilian casualties. It's deeply unfair to blame them for things they didn't do or for decisions they didn't make and never would have made had it been up to them. That being said, there are a couple problems with your argument. First, it's not the case that dozens of intelligence agencies believed the WMD propaganda. They would have been utterly incompetent if they had. They're far from perfect, of course, but no way are they
that stupid and certainly not on that scale.
Rather, what happened is that political leaders pressured the leaders of the intelligence agencies (essentially political positions in their own right) into delivering reports that gave the "right" (i.e. the wrong) answers irrespective of what their professional analysts really thought. Few intelligence agencies could withstand the overt political pressure that was applied to the CIA and others (largely out of the Vice President's office, as I understand it) in the run-up to that war—after all, ultimate power resides with the political leaders in a democracy for obvious reasons. Note, however, that when the same leaders tried to do it again a few years later, enough of the professionals' real opinions was leaked to make a repeat manipulation impossible. I have in mind the unanimous assessment of US intelligence agencies in 2007 that Iran was not seeking nuclear weapons (an assessment that still stands, incidentally, and is still unanimous, though you'd not guess it from the news). When political pressure started to be applied to distort that opinion, someone leaked a summary of it to the press, and that put an Iran war off the table for the rest of the GWB administration. Bush writes about that in his memoirs.
The other problem with your argument is that by the Nuremberg standards, war of aggression is understood to be the "supreme international crime" that encompasses all the evils that take place as a result of the war [1]. That is, the Nuremberg standard of justice considers all the secondary consequences of war (such as Iraq underwent with bombings, civil war, etc.) to be subsumed under the original crime of a war of aggression. This arguably makes sense, since without the invasion, there wouldn't have been a civil war. By this standard, soldiers are not responsible for atrocities they didn't commit, but the leaders who started the war in the first place actually are. Since that is the standard of justice that we (rightly) apply to others, we ought not to reject it or minimize it when it applies to ourselves. (The standard won't be applied in this case, of course, but that's a separate issue.)
The above is how I understand it from the sources I've read, anyway. Factual corrections are more than welcome.
[1] http://en.wikipedia.org/wiki/War_of_aggression