World War I saw widespread use of chemical weapons, causing 1.3 million casualties, of which seven percent were fatal. While chemical weapons were the most feared weapon during World War I, conventional weapons (bullets and shells) where three times more likely to kill. This was scant comfort to the troops, as chemical weapons traveled along with the breeze, collected in the bottoms of trenches and craters, or, like mustard gas, coated vegetation and anything else it came into contact with. Mustard gas caused some 70 percent of the World War I chemical weapons casualties, and is still a popular weapon because it causes terror and demoralization. Touching mustard gas, when it collects on some object, causes burns. Breathing in mustard when it is still in the air can cause permanent lung damage or death.
During the 1980-88 Iran-Iraq war, Iran suffered some 100,000 chemical weapons casualties (10,000 deaths from nerve gas, and 90,000 men injured by mustard gas). The Iraqis began using mustard gas in 1983, and nerve gas in 1984. The Iraqis preferred to deliver chemical weapons from the air, but their pilots often dropped the chemical bombs from too high an altitude (so they missed the target), or too low (so the fuse did not go off and the gas covered a much smaller area.) Total Iranian casualties in the war were about a million, with some 300,000 killed. The chemical weapons were generally used to halt, or slow down, major Iranian attacks. In this respect, the chemical weapons often did the job. While Iraq began the war by invading Iran in 1980, by 1981 the enraged Iranians were doing the attacking and it was chemical weapons that often slowed them down enough to prevent the Iraqi defenders from being overrun.
During World War I, U.S. troops suffered 70,522 casualties from chemical weapons, of whom two percent died (and over ten percent had lifelong health problems.) Throughout the Cold War, the Soviet Union was known to have large stockpiles of chemical weapons and trained their troops to use, and defend against, these weapons. But except the for Iran-Iraq war, there has been no large scale battlefield use of chemical weapons since 1918. Thus soldiers are nervous about what will happen if anyone does use chemical weapons in the future. The two most common, and effective chemical weapons, mustard and nerve gas, require a chemical proof suit to offer reliable protection. These suits have never been used in combat, and all indications are that they would not work well in warm or hot climates. The suits cause enormous and rapid heat build up if the troops are moving around. This means that not a lot of fighting is going to get done, unless the temperature is close to freezing, once the troops suit up.
To add to the soldier's unease, there is the possibility that mustard and nerve agent will be used via a talcum like powder. This approach was used by the Iraqis for their mustard gas attacks in the 1980s. This provided better coverage of the mustard agent. But if you put the mustard chemical on a small enough powder, it can penetrate the protective suits used by most nations.
On the positive side, you have to use tons of mustard or nerve gas on the battlefield to have any effect. The stuff dissipates and degrades quickly. The Iraqis learned this the hard way when they first used it in swamps. A hot sun really does a number on most chemical weapons, frying it into uselessness in short order. If the past is any indicator, an Iraqi chemical weapon attack would likely cause more heat exhaustion casualties among U.S. troops, rather than injuries from the chemicals. This, as you can imagine, does not do much to calm the troops down.
Another morale buster is the poor performance of the "chemical attack detectors." These are devices that constantly monitor the air and give off an alarm if chemical weapons are detected. During the 1991 Gulf War, the U.S. detectors gave off thousands of false alarms. After a while, the troops just ignored the detectors.
The Iraqis would have a hard time using chemical weapons, even if they chose to. Few Iraqi aircraft were able to get into the air, or stay there if they did, during the 1991 war. It's very doubtful the Iraqis will do any better in the future. Iraqi artillery also got shut down quickly during the 1991 war, thus making it unlikely that they could deliver a sufficient amount of chemical shells that way. So if the Iraqis decided to use chemical weapons, the odds of them delivering enough of the stuff to do any significant damage is remote. SCUD missiles are not an effective way to delivering significant amounts of deadly chemicals. And this is the reason why military commanders are not hesitant about facing Iraqi chemical weapons. Of all the things that can get you killed during an Iraqi invasion, chemical weapons are but one of many, and certainly not the worst of the lot.
Chemical weapons are more bark than bite, which is why they are so feared by soldiers. They have never been a decisive weapon, as their main effect is to make most troops in the vicinity lose all interest in fighting. For this reason, many generals were not all that unhappy to see chemical weapons outlawed in the 1920s.