I'll toss my hat into the ring.
9/11 had an effect on the entire world. The only genocide perpetrated by a country that effected the world was the holocaust. None of the others, sadly, had any impact on countries other than their own (save Siera Leone and their diamond trade).
When the terrorists took down the twin towers, it changed the world, not just one country. The US declared war on terrorists and any country harboring them; the economy took a dive, and, not to sound too arrogant, but the world economy is the US economy. Homeland Security and protective measures in airports in the US were adopted overseas. Bombings occured in Great Britain and were threatened in other countries due to an overall plan by Al-Queda.
9/11 was felt by everyone in the world. The holocaust, Chile, and so on? The world took a "who gives a shit? It's not MY country!" defense.
If the US had said "We're going to invade Iraq to remove Sadaam as a dictator, 99.99% of the WORLD would be OK with that. But once that wall drops, it changes completely. The US can't upll out now otherwise we'll have another afghanistan.