is whether invading Iraq prevented a bigger war. GWI isn't applicable because it wasn't a preemptive war.
I'm not sure what Saddam would have done if he had stayed in power. He was an evil bastard. But the US occupation has resulted in well over a hundred thousand documented Iraqi deaths. The true number is likely to be much higher. I doubt the death toll resulting from smacking Saddam around would have been nearly that high. The genocide Saddam committed likely resulted in only about 100,000 deaths.
And I think we need to be aware of the cost of the US's aggressive preeminence too. In that graph for the period of the Vietnam war and connected conflicts, how many deaths were due to US actions. Many accuse us of genocide due to our arguable responsibility for atrocities in East Timor.
Similarly, how many truly large wars have we prevented? There is no adversary for us similar to the Germans of WWII. I don't see our wars really preventing nuclear proliferation.
This is not to say that I think the US is evil. I just don't want to place responsibility for this time of peace at the feet of our belligerence. I think a much more likely cause is globalization: how interdependence is eliminating enemies.
So, I don't see anything in that graph that demonstrates the value of small wars.