Which war? The Iraq War started with around 62% support. When the US started its involvement in the Korean War (one of the biggest mass atrocities we'd carried out since, well, about 5 years earlier when we atom bombed Japan...), around 78% supported it. Around 71% of Americans supported a large scale troop invasion of Afghanistan when it started.
Honestly, even for the wars with bad public perception, like Vietnam, it was mostly because Americans were tired of our guys being drafted just to be turned into dogfood on the other side of the world, not because we were occupying and brutalizing them.
I think the essence of my question is what did "support" look like here?
I can empathise with the position that the invasion of Iraq was warranted (which is not to say that I agree with it), in the context of the September 11 attacks. What I haven't seen is any popular support for the slaughter of civilians or the annexation of territory — there is no grand narrative that the USA is actually liberating its historical lands in Iraq. I think the support was conditional, and based on claims that later collapsed. The end goal was withdrawal after regime change.
What I haven't seen is any analogue to egregious instances like this[0], of which there are many in russia's war against Ukraine.
You're ignoring the mass atrocities committed by both sides in the Donbas throughout much of the 2010s that provided easy propaganda to achieve the same outcomes as the Iraq War propaganda.
I have another example of a "war" carried out recently with overwhelming support in its nation and in the US (initially) due to rapid propaganda around an attack that was likely intentionally intensified in effect by things like moving civilian events next to a military target the day before. But I won't post that one lol
Honestly, even for the wars with bad public perception, like Vietnam, it was mostly because Americans were tired of our guys being drafted just to be turned into dogfood on the other side of the world, not because we were occupying and brutalizing them.