We ought to do what we can to prevent the use of poison gas in the Gulf war; instead, we seem to be unwittingly encouraging it.

When we ratified the Geneva Protocol for the Prohibition of the Use in War of Asphyxiating, Poisonous or Other Gases, the Senate and President Gerald Ford attached an express reservation permitting our use of gas ''in regard to an enemy State if such State or any of its allies fails to respect the prohibitions laid down in the Protocol.'' If we intend to use this right to reply with gas to Iraqi first use of gas on military or civilian targets, we should explicitly warn Saddam Hussein in advance.

Instead, Secretary of Defense Richard Cheney has said that the president would decide what to do if Iraq used unconventional weapons against allied forces, while Pentagon officials have privately told Post reporters that we would not use gas in retaliation against Iraqi initiation of its use. Even if we have already ruled out retaliatory use of unconventional weapons, we should make it clear, at the highest level, that we reserve the right to do so if they are used by Iraq and that any Pentagon speculations to the contrary do not represent U.S. policy.

As it now stands, Saddam Hussein may reasonably believe he can use gas -- and biological and nuclear weapons -- on military and civilian populations without paying a price. We have thus maximized the probability that he will do so. We should not again mislead him with respect to our intentions, as we did last summer before his invasion of Kuwait, when our ambassador reportedly informed him that we had no opinion on his border disagreement with Kuwait. While we will never know whether articulating in advance our present policy of resistance would have prevented his action, we should not blunder similarly again.

DAVID ROBINSON JR. McLean