I'm confused about why so many people say that the Iraq war was based on false pretenses. The main pretense was that Saddam's regime had access to weapons of mass destruction, and chemical weapons fall under that umbrella. The regime had been using chemical weapons against defenseless civilians in the country's Kurdish region, as well as against Iran during the Iran-Iraq war. Given that there is documented evidence of the regime using chemical weapons, which count as a weapon of mass destruction, why do so many people say that the pretense of "Iraq having weapons of mass destruction" was false?
(Mods: my answer uses material that is less than 20 years old, but does cover periods of time that are older than that. Hopefully that's enough to let the question remain up.)
For my response, I'll specifically reference a document published by the CIA in late 2004 called the "Comprehensive Report of the Special Advisor to the DCI on Iraq's WMD." The tldr to your question is that the Bush administration almost certainly knew that at the time of the invasion Iraq was not in possession of WMDs, or that any evidence suggesting it was was likely specious.
Note that my answer does not get detail the case the Bush administration made for the invasion, only why documents like the special report lead people to believe the administration invaded under false pretenses. (The administration's allegations that Iraq was working with al-Qaeda would also largely be discredited later.)
After the end of the first Gulf War, Iraq was required by the UN security counsel to destroy its arsenal of prohibited material related to chemical and and nuclear weaponry, which it ostensibly did. For example, the CIA report states the following relating to chemical weaponry:
While a small number of old, abandoned chemical munitions have been discovered, ISG [Iraq Survey Group] judges that Iraq unilaterally destroyed its undeclared chemical weapons stockpile in 1991. There are no credible indications that Baghdad resumed production of chemical munitions thereafter, a policy ISG attributes to Baghdad’s desire to see sanctions lifted, or rendered ineffectual, or its fear of force against it should WMD be discovered.
(Emphasis theirs.)
The report notes similar findings with regards to nuclear weaponry, however, the assessment regarding biological weaponry states that while they likely destroyed their biological weapons stockpile. they continued to pursue a biological weaponry program when conditions were favorable for it. Nevertheless, by the mid-1990s, due to various circumstances, the CIA concludes that the biological weapons program and interest was all but abandoned.
Despite Iraq's apparent adherence to staying away from WMDs at least since the late 1990s, the UN Security Council felt that the Hussein administration was not living up to many of its other commitments related to ending the first Gulf war. In 2002, for example, the Security Council unanimously adopted Resolution 1441 (PDF) which scolded Iraq for a number of violations, including those related to WMDs:
Deploring the absence, since December 1998, in Iraq of international monitoring, inspection, and verification, as required by relevant resolutions, of weapons of mass destruction and ballistic missiles, in spite of the Council’s repeated demands that Iraq provide immediate, unconditional, and unrestricted access to the United Nations Monitoring, Verification and Inspection Commission (UNMOVIC), established in resolution 1284 (1999) as the successor organization to UNSCOM, and the IAEA, and regretting the consequent prolonging of the crisis in the region and the suffering of the Iraqi people.
The Bush administration cited UN activity like resolution 1441 as one part of its justification for invasion, but ultimately, wasn't able to get the UN to agree that sanctioned action was necessary.
In the end, the findings of the CIA report and the opinion of other governments (like France) boiled down to something like "Saddam Hussein was being a jerk, but he doesn't seem to have had WMDs."
While Hussein had used WMDs against various enemies as you state, they had been specifically abandoned more than a decade before the 2003 invasion in the case of chemical and nuclear weaponry, and partially in the case of biological weaponry (which was abandoned several years later).
The report referenced is quite substantial, and can be viewed in several volumes by clicking to the "Document In Context" on this webpage hosted by the US Publishing Office:
https://www.govinfo.gov/app/details/GPO-DUELFERREPORT/context
A concise, "key findings" version can be found here: