I cannot follow a certain kind of historical argument against the war, which runs like this:
We supported Saddam when he was fighting against Iran. We supported the mujahedin when they were fighting the Russians against Afghanistan. Therefore we are hypocritical, or wrong, or both, to war on them now.
Our past support for both the Taliban and Saddam has been exaggerated, but let’s leave the facts aside and consider the structure of this argument. What, exactly, obliges us always to support someone we have supported in the past? The British fought against us in the War of 1812; are they hypocrites to ally themselves with us now?
Or perhaps you don’t care for reductios. Very well: the Russians were our allies in World War II and our enemies after the war: same government, different circumstances. The shift was neither hypocritical nor immoral nor foolish. The Taliban were useful to us when they were fighting the Russians. So was Saddam during the Iran-Iraq War, when Khomeini was a bigger nuisance. The Russians leave, Khomeini dies, things change.
One can argue that supporting Saddam was a bad idea then. One can argue that removing Saddam is a bad idea now. One cannot argue that the two have anything to do with each other.