I think the repercussions of Iraq have been slow to emerge, but they have been profound. First, a precipitous decline in the moral authority of the US. Second, a collapse in trust in US intelligence. Third, a collapse in the reputation of the US as a reliable and rational partner.

At the time, bear in mind that this was the heyday of liberal interventionism, and people saw Saddam Hussein as a bloody-handed dictator, engaged in sectarian and ethnic cleansing, who was destabilizing the region. The experiences of the NATO intervention in Yugoslavia suggested that this problem could be resolved through regime change. It wasn't possible to know at the time how badly this would turn out.

That’s one read. There is also the idea that lashing out against real or perceived threats to America was seen as justified in the wake of 9/11. And the US squandered whatever sympathy they had by screwing around in Iraq and conveniently ignoring the likelihood that Bin Laden had fled out of Afghanistan into Pakistan.