Victor Davis Hanson asks that question and offers his analysis at NR.
This is my take, in a nutshell.
In what we accepted as the normative terms of geopolitics in 2003, it was right to remove Saddam Hussein and his regime from power. Even the difficulty that followed the early success was nothing that couldn’t be handled. But the normative terms — especially where the U.S. was the status quo superpower and guarantor of strategic peace — were vitally wrong.
The main inference that motivated the regime change was that on September 11, 2001 the era of asymmetrical warfare had begun in earnest and the strategy of conventional containment of Iraq was not a solution to the asymmetrical problem posed by Hussein. WMDs were not the problem. Hussein’s security services with their global reach, his billions in private funds, and his revanchist psychopathy, meant that he could sponsor a hundred 9/11s and probably not leave his fingerprints on any of it. That’s what people who were paying attention were seeing.
This was a strong inference, a valid inference, if the normative assumptions about Iraq and Hussein were correct. But there was a more important date in that calculation than 9/11/01 and that was 1/1/00. That was the date that the KGB, in the person of Vladimir Putin, took visible control of Russia.