Well, technically, if you're interested at all in international law, 'preemptive' wars are deemed legal provided that the aggressor can state suffucient security claims that demonstrate an imminent threat. Since this was obviously not the case in Iraq, you have to conclude that this war was at best 'preventative', which is not legal under international law. (I would argue that the invasion was not even preventative, but, rather aggressive.)
Obviously nobody can stop you from engaging in preventative or aggressive wars, but you have to understand the consequences of ignoring international law. The first casualty is credibility. The second is international cooperation. This might seem all well and good as long as the US remains the top dog in the world, but what if, ten years down the road, this is no longer the case? Do you think it is in the best long-term interests of any country to ignore laws that may ultimately protect you from aggression or to alienate your allies like this?
It's up to you, of course. But as advocates of 'personal responsibility' you need to be willing to consider the consequences of any actions you undertake.
You're making your bed ...