THE IRAQIS, attempting to repulse Iranian counter-invaders, have used chemical weapons -- this time apparently mustard gas -- on at least five occasions in March, the American government reports. Iraq denies it, but some among the estimated several hundred casualties are in the European hospitals where Iran has sent them to be treated and seen. "Some were in severe pain and were just crying out for Allah," a witness to their Vienna arrival reported. "They were a shocking sight."

The war has gone on for years, and neither party enjoys high favor in the West. The effect is an attitude in world opinion holding that the two sides pretty much deserve the worst they can do to each other. There is no great outcry when Iraq -- though Iran also is known to have a chemical capability -- conducts a form of warfare regarded as so horrible and menacing that it has been formally outlawed for 60 years.

This attitude is myopic. Chemical weapons are fairly called the poor nation's nuclear bomb. Five countries had a CW capability 20 years ago, but 14 or 16 do now, and others are shopping around or otherwise making preparations. It may be a bit odd, when you consider all the ways that people have devised to do violence to each other, to worry overly about any particular method. The attempt to restrict this particular form of warfare, however, represents one of the very few organized international efforts to put at least a symbolic civilizing limit on the suffering caused in war.

The United States sees a strategic interest in supporting Arab Iraq and containing fundamentalist Iran. But this political tilt has not kept the Reagan administration from going public, as well as private, with a protest against Iraq's CW policy. It is only by this demonstration of a single standard that a government gains the authority to have its protests heard when its target is an unfriendly government.

Do protests matter? The Reagan administration in its first term led an international campaign criticizing the Soviet Union for the use of chemical agents by Soviet troops invading Afghanistan and by Soviet- supported Vietnamese troops fighting elsewhere in Indochina. One cannot be laboratory-sure of cause and effect in these situations. It is noteworthy, however, that there have been no confirmed reports of chemical use in Indochina for at least two years and in Afghanistan for one. It may not help immediately or directly, but it cannot hurt for the Iraqis to be held up to obloquy and censure for the use of gas.