> Hussein wanted the West to believe that he had no chemical weapons.

There's a standard joke that may be close to the truth. The joke goes, a US official said: "Of COURSE we knew that Saddam had chemical weapons because we still had the receipts from when we sold those weapons to him.", rough quote. The implication was that the US sold chemical weapons to Saddam for Saddam to use against Iran in the Iraq-Iran war.

In Gulf War II, George W. Bush, Bush 43, invaded Iraq, occupied the country, captured Saddam, and let some Iraqis put Saddam on trial and hang him, all for whatever reasons Bush said or believed and didn't say or whatever.

There are various candidate reasons; here are 10 such:

(1) The attack on the US on 9/11/2001 early in Bush's first term made Bush eager to defend the US against terrorism. There were claims, and maybe Bush believed them, that Saddam was working with Al Qaeda on more such terrorist attacks on the US.

(2) There were claims that Saddam had started to make biological and/or chemical weapons of his own and so hated the US that he might try to deploy such, via terrorism or whatever, on the US. Bush wanted thorough inspections of Iraq, but Saddam didn't cooperate which made Bush suspect that Saddam was trying to get and deploy such weapons against the US. Part of the issue of did Saddam cooperate were claims that Saddam had shipped biological and/or chemical weapons to some valleys in Syria to evade US and/or UN inspectors and later to retrieve the weapons for use against the US.

(3) Like (2) except for nuclear weapons.

(4) Saddam was a brutal dictator using tactics borrowed from Stalin. Maybe Bush wanted to stop that.

(5) Saddam used chemical weapons against the Kurds in Iraq. Maybe Bush wanted to stop, retaliate against Saddam for that, or conclude from that that Saddam was so evil he should be killed.

(6) Saddam was not obeying the rules stated at the end of Gulf War II.

(7) Saddam had some sons who were nasty.

(8) There were claims that Saddam tried to kill Bush 41 and that Bush 43 wanted to retaliate.

(9) Bush 43 saw Saddam as being cruel to the people of Iraq, said that "The Iraqi people are perfectly capable of defending themselves", and maybe wanted to make that possible. For the invasion in Gulf War II, the US military did capture Baghdad, occupy the country, and guide Iraq to create a new constitution and hold held national elections; the US did all of that quickly. Maybe then Bush thought that the US could leave Iraq quickly, but, no, there was an insurgency, and essentially it is still going on.

(10) Saddam built several palaces in Iraq, and maybe Bush thought that such palaces were improper and that Saddam should be killed for that.

At the end of Gulf War I, US General Schwarzkopf knew that he could take Baghdad and occupy Iraq, quickly, especially if the rest of the coalition would go along but did not want to do that and believed that the coalition would not go along. Five years later in an interview in a two hour program by Frontline, Schwarzkopf said that if the US had occupied Iraq, then that would have been expensive "and, by the way, we would still be there", IIRC. Yup. And we still are.

In Gulf War I, Schwarzkopf, and likely essentially all the coalition, wanted (A) to drive Iraq OUT of Kuwait and (B) to destroy the Iraqi military as an offensive force and, in particular, destroy or weaken the Iraqi Republican Guard, a main pillar of Saddam's power in Iraq. Schwarzkopf accomplished (A) thoroughly and (B) well enough for himself and, then, wanted himself, the coalition, the US military, and the US OUT of Iraq, that is, just leave Iraq to the Iraqis for whatever outcome that might be, pretty, ugly, whatever.

So, in short, at the end of Gulf War I, Schwarzkopf clearly saw that for the US to occupy Iraq would be a big mistake. But later Bush 43 did just that.