Don't underestimate the psychological effect of the mere threat of chemical agents. When the South Africans discovered the racks for storing chemwar agent containers in Namibia it triggered huge spending on chemical warfare protection. Something like this could happen to the "Coalition" in Iraq.

Just the threat of chemwar agents such as Phosgene (carbonyl chloride; which had pretty much replaced Chlorine as a chemwar agent by the end of WW1) or the simpler nerve agents (Tabun or Sarin) or blistering agents like HS/HT or HN could have troops in CBW gear, reducing their ability to operate. Just imagine wearing a rubberised suit in Iraqi heat....... Not to mention panicked troops self-medicating with Atropine. Even the threat will elevate stress levels still further.

It wouldn't be particularly difficult for insurgents to produce small batches of nerve agents, especially in a country with plenty of open space for such efforts.