Some say it was about oil, but read The Iraq War Was Not About Oil. Maybe some Senators voted for the war thinking that oil might make it worthwhile, but the war was never waged in a way to get an oil benefit, and we never did.
Some say it was about lies over WMD, but most of those people seem to be unaware that WMD were found in Iraq. See the Wikipedia article on Iraq and weapons of mass destruction. George W. Bush and Tony Blair argued that Iraq was not complying fully with weapons inspections, and in fact it did turn out that Iraq had chemical and nuclear weapons materials that were not exposed until after we invaded.
These materials were not an imminent threat to America, but I don't think anyone said they were.
Nor does it make any sense to say we went to war to contain militant Islam. Other countries were bigger Islamic threats. Wikipedia reports on Christianity in Iraq:
The Christians of Iraq are considered to be one of the oldest continuous Christian communities in the world. ...So obviously the war was a disaster for Christianity in the region.
In Iraq, Christians numbered about 1,500,000 in 2003, representing just over 6% of the population of 26 million (down from 1.4 million or 8.5% of 16.5 million in 1987). Since then, it has been estimated that the number of Christians in Iraq have dropped to 500,000+.
Christianity and oil are two of the pillars of Western Civilization. If we had to fight a war to defend those pillars, then that could make sense. But apparently our leaders who waged this war had no intention of defending either.
The United States has spent nearly $6 trillion on wars that directly contributed to the deaths of around 500,000 people since the 9/11 attacks of 2001. ...Those 9/11 hijackers could not have realized that their big accomplishment was not bringing down the WTC, but suckering us into spending $6 trillion, and into advancing the Islamic cause in some ways.
In February, President Donald Trump estimated that "we have spent $7 trillion in the Middle East," saying "what a mistake" it was.
I also wonder why we fought World War II. Many say it was a necessary war to prevent Hitler from taking over the world. But read The Lies About World War II. It says that England and France started WWII, and the war was unnecessary and stupid. I am not sure, as it is hard to get objective info on the subject.