There's a half-remembered folk history of the lead up to the invasion of Iraq that's shaped the war's modern understanding in the United States. Here, I'll lay out in brief the several contributing factors that made the invasion happen.[1]
They Thought It Would Work
This is sort of silly to bring up because it applies to most everything that has ever happened in history: people do things because they think they'll work out, the risks will outweigh the benefits, and in the end they'll be better off than how they started. But I want to discuss this here because the modern discourse isn't always interested in understanding the perspective of the perpetrators. And it's not just Bush and Cheney that thought it would work: the war happened because there were multiple constituencies or interest groups who thought it would work and these groups drew their own benefits, often different from each other, from the war. The Republican party, just like the Democratic party, is pluralist: there are multiple groups with their own interests and can have different motivations for the same action. That's mostly what we'll discuss here: who are those groups and what did they see in Iraq? Ultimately, what made the US invade Iraq was this confluence of interest groups all seeing an advantage to what Dubya was proposing.
The Same Team Already Did It Once
They thought it would work because the US already tried invading Iraq once before in the Gulf War and that went pretty well. Some of the same people came back to do it the second time, notably Dick Cheney (SecDef) and Colin Powell. Maybe this mostly reassured the administration itself but you can see how they might go into the war with the mindset that it'll work out just like the Gulf War.
Cultural Conservatism
Remember that Republicans are genuinely conservative: they hold conservative political beliefs, they think those beliefs are consistent and correct, they want to see a country and world oriented to their conservative views and they actively work towards those goals. I want to dispel the sometimes implicitly held idea that these beliefs are purely rhetorical, just a fig leaf over someone's genuine political motivations. Maybe this does not apply to everyone but does apply to lots of Republicans, especially once you get away from politicians and down towards the base. For the duration of the Cold War these conservatives had a consistent message: the US stands against Communism and authoritarian governments. An invasion of Iraq gives a renewed version of the branding: the US still stands against authoritarian and oppressive governments and is willing to use its industrial and military might to be a force for good in the world. It gives the masses something concrete to believe in and a movement to belong to. A war reaches all corners of the society, it gives an opportunity to rally around the flag in public, and it can even reach down into the smallest rural communities when their young men enlist, potentially giving a personal connection to the far-away conflict. The war becomes a way to build the conservative culture that you wish the country had.
Saddam Was A Bad Dude
The moral aspect of overthrowing Saddam resonated with the public because there was a sort of cosmic background radiation about the guy from the events of the 80s and 90s. Saddam had use chemical weapons against his own people before! He attempted genocide against his own citizens in 1998. Even Bernie Sanders, who famously opposed the 2003 invasion of Iraq was in favor of overthrowing Saddam in a speech in 1998. Bernie is not being inconsistent here: he believes the US should defend the oppressed people of the world, he just differs on the details of how it should be done. In the 1998 speech he's arguing the US should be arming an anti-Saddam insurgency and in the 2003 speech the US should operate within a UN coalition instead of unilaterally. Even when the arguments made by the Bush administration for the 2003 invasion flopped it wasn't enough to shake popular support of the invasion because the country was already pre-positioned to accept that overthrowing Saddam was just.
Stovepiping
We now know that the evidence presented for Saddam's WMD program or collaboration with Al-Qaeda was wrong. Today there is a common belief, either stated or implicitly held, that the entire government intelligence community was behind this evidence and Iraq became either the first instance or the big reveal of the intelligence community's incompetence. In other words: they got it wrong with Iraq, why would you trust them on the issues of today? But even at the time there was widespread criticism of the Bush administration's evidence both outside and inside of the government: for example, the Plame affair can only happen as an act of revenge if someone is credibly and publicly criticizing the government's position. The administration had to turn to tricks like the Office of Special Plans, a whole institutional apparatus created just to shortcut the existing intelligence community to give Bush the evidence he wanted for the invasion. This stovepiping technique is insidious: it hijacks the credibility of the existing organization and uses it to sway those who trust the institutions with a work product ultimately not created by the institutions. Personally, I think stovepiping contributed to war support from a lot of people who "should have known better" like the Democratic leadership at the time: they were victim to an intentional plan to take advantage of institutional trust. It would have been better if they trusted institutions a bit further and leaned more into the non-stovepiped arguments that were made but unfortunately that didn't happen.
The End Of History
There was a well-timed argument made in 1989 that liberal democracy won out over authoritarian governments and that going forward the authoritarian governments of the world would eventually collapse out of the popular preference for democracy. Francis Fukuyama, the author of this "End of History" argument, eventually falls in with the like-minded 90s think tank Project for the New American Century. The PNAC becomes the spawning ground of neoconservative thought and Bush drafts many of its participants into his administration. That's the history, but I want to convey some of the mindset of these neoconservatives. Coming into the 90s they were greeted with a validation of their belief in liberal democracy and capitalism with the collapse of the USSR. Under George H.W. Bush the Gulf War was an incredible success, once again validating their faith in the US military. As the 90s dragged on the economy took off, eventually becoming the dot com bubble but genuinely transforming business and society for the better with improved computer and communication technology. The PNAC was high on its own supply, seemingly validated at every turn, and had no problem pitching aggressive and risky plans like the overthrow of Saddam Hussein. The neoconservatives were the constituency of fart-sniffing true believers.
Shortly after 9/11 George W. Bush gives a speech where he famously pitches that Al Qaeda "hates us for our freedoms". I want to bring this up because I think Bush is actually making an "End of History" argument here in a more digestible form for the public. The claim on its face is not true, 9/11 and Al Qaeda were motivated by Israel, but Bush's statement presupposes the End of History desire for liberal democracy and turns it into the motivating force for Al Qaeda. Here, the neoconservative ideology gives a way for the the administration to sell its ideas to the public. The same thing happens with "we will be greeted as liberators" - once Saddam is removed the End of History-driven desire guarantees the Iraqi people will self-organize a democracy.
Defense Contractors
A popular idea at the time was that defense contractors, looking for government war contracts, demanded the invasion. I think the military-industrial complex is certainly one of the groups in the coalition in favor of war but the popular conception that the defense industry dictates what it wants out of the government is backwards. I will elide some detail here for a later post but I think the appropriations and bidding processes keep civilian control over what contractors build and how much profit they can ultimately make. Ultimately, the Defense Production Act hovers over everyone's head as well: if things get real bad, the government can simply say "thank you for playing" and nationalize your business, wiping out your chance at profit.
Oil
Oil is important but I don't think the administration was motivated to invade Iraq for its oil. Iraq didn't have the oil, Kuwait did! That's why Iraq invaded Kuwait in the first Gulf War after all: Iraq's oil industry was economically uncompetitive. The US also started its invasion with the destruction of Iraq's oil industry infrastructure which is not what you'd do if you want to start exporting their oil. Dick Cheney, immediately before he was Vice President, was CEO of Halliburton, a company that originally provided oil infrastructure services but moved into defense contracting. Halliburton did secure oil and defense contracts from the US government during the war and Cheney and Halliburton's other stockholders profited from it. I don't think war profit was the motivation for many, though: it is just one of many causes that can help move some constituents to support the war. Cheney, in particular, I think is largely motivated by his neocon ideology even though he also stands to profit substantially from the war.
Revenge?
Iraq did attempt to assassinate George H.W. Bush, the father of George W. Bush, and some have floated the idea that the invasion of Iraq was motivated by GWB exacting revenge on Saddam for the attempt. Maybe? If could be in this big pile of motivations. But even if it was, it motivates only one person (albeit a very important person): GWB himself. Maybe it helped grease the wheels a bit but I don't think it contributed much.
Israel
Saddam Hussein had no way to strike the United States: he has no intercontinental bombers, no international navy and no ICBMs. Despite what the Bush administration suggested in the lead-in to the war Saddam had no capacity to attack the domestic United States. Israel, on the other hand, is a closer target and Iraqi-Israeli relations are not great. After the first Gulf War the UN imposed strict limitations on Saddam's missile program, limiting him to missiles unable to strike Israel, and these limits were enforced by UN inspections for some time. But it is not unreasonable to suspect that one day Saddam might acquire missile technology with the range he needs. Israel fears a war against all of its neighbors simultaneously (not that strange of a hypothetical, it's happened before!) and an Iraqi military with its population, relative military competence, and missiles that can strike from Iraq make it a serious threat. If Saddam does resurrect a chemical, biological or nuclear program he can launch dirty bombs at Israel. If he was willing to use dirty bombs against his own population of course he'd be willing to use them against Israel! You can see how Saddam Hussein and his military are a much larger threat to Israel than they are against the United States. A war that both overthrows Saddam, destroys his army and destroys any WMD labs motivates the pro-Israel constituency in the United States in favor of the war.
Kitty History
Everything I wrote above is bullshit. Here's why it really happened.
[1] Shoutout to Why Did We Invade Iraq? by Charles Davis which argues basically the same thing I am doing here but a bit more thoroughly and rigorously. I found this article while doing research for this one.