WE'RE PLEASED to welcome Ian Smith, prime minister of the besieged interim government of Rhodesia. The State Department finally decided to let him in, and, to judge by its public statements, for just the right reasons. The America people will be able to hear directly from one of the principal parties to a conflict in which American diplomacy is involved up to the ears. American officials will have further opportunity to consult with Mr. Smith, and with other members of the Salisbury government's executive council, on ways to bring Rhodesia's blazing war to a peaceable end.

To make his visit mutually useful, the various people involved in it have got to perform in a certain way. Here are our suggestions:

First, Mr. Smith. He evidently hopes to appeal to the public and the administration's critics to force the administration to recognize the settlement he made with "internal" black nationalists last March. Good luck. But if he really thinks he can do that, he is grievously misinformed about the American political scene. He should not mistake the certain sympathy that his regime commands for the broad and deep support he would need to turn the administration around. He must realize that the fair-play impulse that led many people to want him to have the chance to make his case is something quite apart from a desire to see his case prevail.

The senators who invited Mr. Smith, and other Americans who have felt the administration was tilting toward the guerrillas, have their own responsibility. They should not lead him to believe that if he hangs on he will be bailed out. To promise Mr. Smith recognition and the lifting of sanctions is to play a cruel trick. Change of that sort is not in the cards. Given the administration's basic thrust and the relatively short time left for the Rhodesian tragedy to play out, the change that can be expected is much more limited. Support for yet another go at a deal between Smith and Front leader Joshua Nkomo may be about it. There can be no guarantee that the administration can deliver even that.

Then there is the State Department. It pleads it is playing Rhodesia right down the middle, and cites the fact that both sides complain to prove its point. But it has not proved its point. The guerrillas of the Patriotic Front may complain but they feel they are winning; Salisbury complains and fears it is losing. This crucial difference in perspective is ignored. The Front currently is aboard the State Department's proposal to arrange a transition to free elections at a conference, or so the department says. Mr. Smith and his colleagues don't trust the department enough to accept its proposal. Precisely there, in breaking down Mr. Smith's distrust, lies the department's principal task. How can it be done? Providing the answer is what diplomacy is supposed to be about.