THE downing - by United States missiles - of Iran Air Flight 655 in the Persian Gulf, a civilian flight with 290 aboard, was a tragic mistake. President Reagan's initial expression of ``regret'' should be followed by an apology to Iran and an offer of compensation to the families of the citizens of Iran and other countries lost in the disaster.
Whatever the contingencies - including a skirmish between two US Navy vessels and three Iranian gunboats some 12 minutes before the missile cruiser Vincennes fired on the passenger aircraft - the downed aircraft was a civilian flight, with 52 women and 66 children aboard.
The United States government does not excuse the shooting down of civilian aircraft, whether its military officers mistake them for fighter planes, whether equipment fails to discriminate accurately enough, or for any other reason.
An inquiry will go over the events and procedures. Whether an adequate warning had been issued by the Vincennes to Flight 655 will be checked. Through this review, fairness should be extended to the Vincennes' skipper and crew, who themselves may have been caught in an impossible challenge - protecting their own lives in a theater of undeclared war too small for a reasonable margin of error.
The immediate issue is to clear up the matter of Washington's intentions. Iran need not react, as its government has threatened, to an imagined act of hard-hearted hostility.
US policy for the moment need not change. It might have been better, at the outset, not to have promised Kuwait that the US would guarantee safe passage through the Gulf. But on that promise the Reagan administration must make good.
Speculation divides on whether the downing of Flight 655 will galvanize a floundering Iranian leadership or hasten its disintegration. Either way, Washington remains responsible for its own conduct and decisions.
Comparisons with the downing of a South Korean airliner by a Soviet fighter plane five years ago do not hold. The time frame was different. The Soviet fighter plane was in pursuit far out at sea, with time on its side; the Vincennes was sitting in the water, potentially under attack. And so forth.
Still, using even the most ``sophisticated'' arms and technologies to police zones of conflict bears risks to innocent persons. Mistakes can be made. This Washington must concede.