I think we have to keep in mind that the american "left" is a baby that has only existed since Bernie's 2020 campaign. Everything before that time has been filtered through the liberal worldview. Any americans that were old enough to be an adult and switched on to politics in 2003 probably supported it but felt there were "bad actors" among them but the troops are mostly good.

Additionally the only filter they got that information through was liberal media of 2003. It's long long before social media which is showing the criminality of war more than the liberal filter of television would do in the past. If Israel's actions today happened in 2003 almost nobody would know about the genocide and all the Israeli narratives would've succeeded. The media wouldn't have had to show anything bad at all, they would've hid it all.

So we're dealing with a left that is just 6 years old and only has experience with those 6 years. Everything else doesn't exist or has the distorted worldview. This will change, especially when the US has to put troops somewhere and the conduct of those troops shows they're indistinguishable to the IDF.