Averell Harriman's birthday party (88, if you want to believe it) began last night with a grim film called "Nuclear Nightmare," which was about "the wars that cannot be allowed to happen."

Grim. But as Harriman said afterwards, "the control of the grim" is now very much what Americans must work out.

"When I was a boy," he said, "we thought we had a world of peace."

Victoria reigned. Railroads ran. Progress was everywhere and even if kids worked in the mines, still everybody knew the world, and specifically America, was getting better and better wihtout any end.

And now. Now. He is an old man, far past the usual shuffling for position, and well past ambition.

He spoke of the forthcoming presidential race, observing he never lost a friend merely because of difference in politics (there were Republicans present).

And yet, he went on, not one thing in the coming year is so important as SALT II. Any politican, he said, who takes on SALT to try to make jingoistic political hay from it, is going to be a dead duck. Because Americans he said, know it is important and must be ratified.

"I wish I could believe that," said one -- said a number, probably.

"I thought I was going to a birthday party," whined one fellow after the film.

"It started off a downer and there was no way to go but down," said a gorgeous woman who was nevertheless keeping her strength up at the buffet.

"I thought it was excellent," said Stu Eizenstat the presidential adviser.

"So did I," said Landon Butler, the handsome Tennessean who is Hamilton Jordan's deputy.

Michael Forrestal, chief executive officer of the Soviet Trade and Ecomonic Commission, was down from New York:

"He's right about SALT. I deal with the trade people in the Soviet Union. If SALT is not ratified, they tell me, they're out. Only the hard-liners will be left."

Pamela Harriman, always a handsome woman, was excited and radiant at the party. The evening was clearly a vast success, with a great turnout of the Establishment.

Sen. Charles Percy (R-Ill.) was supposed to introduce Harriman but when he was detained at the Senate, Harriman took the liberty of introducing himself.

Sen. Alan Cranston spoke birthday praises at the dinner, followed by Zbigniew Brzezinski, who briefly reviewed significant points of Harriman's long career.

"He's going on too long," said a fellow.

"But he has brains and wit," said another.

"He should be using them," muttered the first.

Someone speculated where the secretary of state, Cyrus Vance, was.

"Up to his ears, I would think," said a somber man, "with Iran."

Among the guests invited were Joseph Califono, John Gardner, Henry Catto, Sen. John Chafee (D-R.I.), (his wife sensibly filled her plate and left him to catch up with her), the Richard Helmses, Sen. and Mrs. Mark Hatfield, Kay Halle, Marvin Kalb.

Harriman thanked everyone for coming. Many must have thought of the time he accompanied Chruchill from England to see Roosevelt -- Churchill was afraid the Germans might develop an atom bomb, and the Manhattan Project was born. And of his work on the limited test ban under Kennedy. All those years.

The spectrum of the capital ran on through Luvie Pearson, Evangeline Bruce, the Maurice Tobins, Paul Warnke, Anne Wexler.

Antonia Chayes, assistant secretary of defense, sometimes raised her hand to her cheek during the film. She thought the film might be valuable, perhaps if edited . . .

An officer from the Pentagon said an hour and a half was simply too long.

The producer, Tom Johnston, said his main worry had been to strike a balance between grimness and human interest sufficient to keep people from tuning out in despair. The film is under the aegis of the British Broadcasting Corp. and an American public television station, and will be aired later on public television.

"I certainly agree with it in principle," said a serious young man. "I don't guess many people are in favor of nuclear war."

Brzezinski said he thought it was too long, and should be edited much shorter.

"Maybe half an hour?" he was asked.

"Yes," he said. "Like a nuclear war."