According to an entry in Wikipedia, "A crime against peace, in international law, refers to the act of military invasion as a war crime, specifically referring to starting or waging war against the integrity, independence, or sovereignty of a territory or state, or else a military violation of relevant international treaties, agreements or legally binding assurances.'' As per the definition above, the war against Iraq by the US led so-called "Coalition of the Willing" qualifies in no small measure as indeed a crime against peace. It may be further noted here that the next paragraph of the Wikipedia entry reads, "An important exception to the forgoing are defensive military actions taken under Article 51 of the UN Charter.''
The basis of the launch of the war (to provide the pretext required to circumvent the "crime against peace" charge), was that the Saddam Hussein regime possessed "weapons of mass destruction" which was always known to be bogus, and was covered with the fig leaf of "intelligence failure". However, what is also known is that the Bush administration in the US has been suffused with enthusiasm for a doctrine of preemptive strike and regime change and for the use of massive force against an inimical government with nary a thought for the peoples of the country. Therefore, it may be readily concluded that it would not be possible for the Bush administration to credibly hide behind the exception referred to earlier.
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For the latest demonstration of "the gap between what President Bush and members of his administration were saying publicly during the run-up to the war and what they were saying, and doing, in more private settings, " take a look at the Crawford Transcript, and the accompanying commentary by Mark Danner.