David Frum offers one reason why war supporters were so overconfident in 2002-03:

But while support for the war was broad, it was also shallow. Members of Congress, public opinion – and I myself – were all over-influenced by the recent Serbian and Afghanistan experiences, which seemed to promise that bad regimes could be toppled rapidly and easily by remote weapons.

Had the president asked Congress for a trillion dollars to fight a war that would last seven years, Congress would have recoiled in appalled dismay.

I’m not sure that this explains very much. It is almost always the case that there would be much less support for a war if the full costs were known before it happened. War supporters routinely promise that this time the war will be easy and successful. They are almost always wrong, and even when the war is relatively easy and successful it is usually much less so than they predicted. It is in the nature of advocates for new wars to minimize and underestimate the risks of conflict, which often involves misleading the public and offering bad analysis, and it is in the nature of war to discredit the sunny predictions of warmongers.

It’s worth remembering that the U.S. and NATO waged the war against Serbia in 1999 on the assumption that Milosevic would capitulate in a matter of days. Instead, they ended up with a bombing campaign that last ten weeks. By the end, they were contemplating launching a ground invasion, which they might have had to carry out had it not been for Russian pressure on Belgrade to yield. Even then, Milosevic wasn’t toppled as a direct result of the Kosovo war. Milosevic lost power well over a year later because of the efforts of the local political opposition. The experience in 1999 showed that a country’s population rallies against foreign attackers even when they may hate their leadership, and it proved that even “easy” military interventions drag on much longer and tax U.S. and allied resources more than anyone anticipated at the beginning. Nothing about that should have made hawks overconfident about waging a much larger, riskier, more complicated war to overthrow and then replace a foreign government as an occupying power. Driving Serbs out of Kosovo and using the Northern Alliance to push the Taliban out of power had very little in common with the U.S. and allied invasion that the administration was proposing in 2002-03.

In fact, pro-war advocates rarely invoked recent U.S. military successes when trying to explain why the war should be fought. They invariably referred to U.S. victories in WWII and went out of their way to compare the proposed invasion to that war effort. Some version of “we did it in Germany, we did it in Japan, and we can do it again in Iraq” was the rallying cry. They sometimes also made far-fetched comparisons between post-Hussein Iraq and post-communist Europe to “prove” that rapid democratization after the fall of an oppressive regime was going to be easy to achieve. They were substituting nostalgia for analysis, as if reciting slogans about success in WWII and the Cold War ensured success now. They were brazenly ignoring the huge differences between the countries in question, since there was nothing of less interest to them than the distinctive culture and history of the country they were proposing to “transform.” Worst of all, many of them were indulging an idealistic fantasy that a functioning democratic state would more or less just emerge from the ruins of a brutal authoritarian system of its own accord.