We can bomb Iraq. We can kill Iraqi soldiers in combat. We can overthrow Saddam Hussein's murderous regime. But we can't use tear gas? So it would seem according to international law.
Under the 1993 Chemical Weapons Convention, which the U.S. Senate ratified in 1997: "Each State Party undertakes not to use riot control agents as a method of warfare." This means using nonlethal chemicals—i.e. tear gas and pepper spray—to subdue enemy troops is a violation of international law. Even if such an act was used in order to minimize non-combatant casualties, something which is a concern in Iraq, where paramilitary Saddam loyalists are taking refuge in civilian areas.
There's also the question of why the Convention permits the use of nonlethal chemicals for "riot control" purposes, but not as a means of warfare. Practically speaking, most nations would have been reluctant to ratify the Convention had it attempted to dictate domestic law enforcement practices. Still, it's ironic that we can use tear gas against anti-war protesters, but not against the regime we're actually waging war against.