According to the latest batch of Wikileak documents. In particular, coalition troops kept coming across numerous artillery shells filled with mustard gas.
Remember the terms of Section 8 of UN resolution 687 that ended the first Gulf War:
Iraq shall unconditionally accept the destruction, removal, or rendering harmless, under international supervision, of:
(a) All chemical and biological weapons and all stocks of agents and all related subsystems and components and all research, development, support and manufacturing facilities
Thus the instance of a single chemical weapon in Iraq violates the terms that ended the Gulf War, and thus the decision by George W. Bush to carry out the liberation of Iraq in 2003 was both legal and justifiable. (This is in addition to the other numerous instances of jus ad bellum cited by Bush in his speech on that reasons behind that decision.)
Liberals, look at yourself in the nearest mirror and repeat after me: “George W. Bush was right and I was wrong. George W. Bush was right and I was wrong.”