We were likely to have gone to war with Iraq even if 9/11 didn't happen.
We could have done it better, and less agressive, but we needed to do something.
Iraq had lost it's sovereignty as far as a state can do under international law, there are 4 conditions which must be meet for a state to lose its sovereignty these are: 1. Invading a neighboring country 2. Violating the Genocide Convention 3. Harboring terrorists 4. Violating the Nuclear Non-Proliferation Treaty
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A country destabilized and countless civilians displaced is not enough to legitimize a war that was hastily waged.