The invasion of Iraq was a renegade act partly because of the precedent it set in international affairs.
Could not North Korea preemptively attack South Korea (which the North has recently threatened to do), since it contends (and might very well believe in its state-paranoia) that the South has active plans to attack the North? North Korea argues it would simply be protecting itself to launch a first-strike nuclear attack.
From this, it is all too nightmarishly easy to see that relations between nation-states remain dangerously anarchic because of the advancement of the concept of preemptive invasion (of Iraq).
The invasion of Iraq was "illegal" because it lacked full United Nations support, support which the U.S. claimed it did not "need" (thanks to reckless Dick Cheney, et al.).
But, what we and the rest of the world need is an international system that insures mutual safety, not single-state self-protection that ramps up the edginess (and likelihood) of warfare. We do not want a return to pre-World War I international competition and rivalry and exclusive (or even interlocking) self-protectiveness.
This is exactly the reason that Pres. Obama--for his rationally peaceful rhetoric--was awarded the Nobel Peace Prize before he actually did anything much more than talk. International talking--i.e., diplomacy--might not be sufficient, but it is a requirement.