Once upon a time someone gave a party. It should have been a great success. The drinks were cold, the food was hot, the house was full of flowers, and the guest list was a pleasant mix of the solid and the eccentric, including one playwright, one newspaper reporter, a political scientist with an open mind, two priests (one of whom brought his guitar and a sheaf of antiwar songs) and a lawyer who knew Marlon Brando personally and had just flown in from L.A. But things went wrong.

The lawyer, suffering from jet lag and gin on an empty stomach, got into an overheated argument with a guest who was certain Brando had played Nathan Detroit in "Guys and Dolls." He didn't, said the lawyer.

He did, countered the guest.

He didn't, said the lawyer, who slammed half-sober into the kitchen to telephone Marlon in Topanga Canyon to straighten the whole thing out.

Meanwhile, the playwright got on the wrong side of the reporter over Reaganomics. The antiwar priest, overwhelmed by capitalistic vibrations in the room, retired to a corner to play vaguely critical songs such as "Hey, Mr. Businessman." The political scientist never did find anyone to talk to, and when the lawyer returned to admit his mistake about Brando, almost everybody had gone home.

The hostess, who had watched the evening deteriorate like a bad play out of control, wrote notes of apology to her guests the next day. The guests protested. They had had, they claimed, "a wonderful time." But they meant "memorable," which is not the same thing. The Vietnam War was memorable, but at bottom, it was the fruit of failed diplomacy. And at its highest, most rewarding level, a successful party is a successful diplomatic act.

Washington is full of diplomats, including 88 ambassadors, although they mostly remain out of common view. A diplomat (once defined as "an honest man sent abroad to lie for his country") is, beneath his pin-stripes, a master plumber, hired by his government to make sure that its relationship with the host country does not spring any leaks. At the highest level, the ambassador often does this at his diplomatic residence, at a party, where the soothing solvents of wine, food and flowers take the edge off the negotiating table and pave the way for understanding the other person's point of view.

Each country has a rank, tied numerically to the date on which its current ambassador in Washington presented his credentials to the president. The Soviet Union, for instance, is DPL-1, should you happen to see that license plate in the Sears parking lot. Sweden is DPL-10.

But these are just numbers. True clout lies with the most powerful or most strategically committed countries. Thus, Norway, a sliver of land on the Atlantic, makes greater waves here, or could, than Sweden, its larger neighbor on the east. Norway is a member of the North Atlantic Treaty Organization and has oil. Sweden is a neutral power and, as one former American diplomat wryly observed "has reindeer."

But since 1974, the Swedish Embassy in Washington has had Count and Countess Wilhelm Wachtmeister as an ambassadorial team. The Wacht- meisters entertain more or less endlessly. They do it extremely well.

They lead on the surface, charmed lives. Count Wilhelm Wachtmeister is tall, handsome, hardworking and a member of an old Swedish family that has produced diplomats and public servants in almost every generation, like the Hammarskjolds, another, old aristocratic family. Dag Hammarskjold, Wachtmeister's superior at the United Nations when Hammarskjold was secretary general, died in 1961 in a plane crash in what is now Zambia, where he had flown to negotiate a ceasefire in the Congo civil war. Had it not been for a last-minute need for Wachtmeister to remain behind in New York to be in charge of the General Assembly, he would have shared Hammarskjold's fate.

Countess Ulla Wachtmeister is blond, beautiful and genuinely self-effacing, prone to protesting that she is "terribly spoiled." This opinion does not seem to be shared by anyone else. She is a worker, a perfectionist, someone who decides to dig up a dozen giant rhododendron plants and pot them in Chinese urns for a party, and does it with very little help. She runs the embassy with a full-time staff of three (a butler, a housekeeper/second butler and a cook.) It is a tossup as to who is the most crucial member of the Swedish embassy, Count Wachtmeister, his wife or Mr. Johansson, the tall, craggy-faced, blue-eyed butler who moves efficiently between them.

"Willie," as the Count is called on the tennis court, is the politician. At the various, light-struck parties they give, he generally prefers to hole up with a few key guests in his library. It is a dark and profoundly decorated den of thought, filled with oil paintings, Russian icons and a scattering of photographic talismans of power reflecting who is in power now. A photograph of Vice President Bush is on a table near a window, with the light shining full on his face. Jimmy and Rosalynn's autographed picture is behind a sofa, competing with a lamp.

The Wachtmeisters know everybody, from Liv Ullman to the director of the FBI. When Ingrid Bergman came to town, they entertained her, as they have entertained the Kissingers, the Brzezinskis, poets, painters and royalty. Yet their parties look as artlessly unplanned as a fistful of buttercups, and if Willie is the man of substance, Ulla is the grace note, weaving lightly through the crowd.

"Other ambassadors' wives," remarked one observer, "are just as competent, and some--like Mary Henderson (wife of the British ambassador) are far more political. Ulla never grills anybody about their political views. But the main difference between her and all the others is her friendliness and her taste in combination. Both are genuine and everlasting. I do not know her well. No one does. But I am immensely fond of her." Everyone interviewed seems unabashedly to share that view.

Sweden is not a simple country. It is a welfare state with a royal family. It has been neutral since the Napoleonic Wars nearly forced the Swedes into the Baltic Sea. But it is well-defended; the Swedish air force rivals the French.

A land of lakes and forests, Sweden is far less dependent upon the export of lingonberries than high-tech software and computer parts. And while the Swedes themselves are shy, Ulla Wachtmeister is a warm fire in the grate and in the seven years that the Wachtmeisters have been in Washington, Ulla Wachtmeister has turned Sweden's image of being a small, faintly self- righteous country on its ear.

Entertaining is a key to unlocking the door that, for a brief moment, during the Nixon years and before the Wachtmeisters arrived, had been slammed shut.

In 1972, Olaf Palme, Sweden's prime minister at the time, made a cutting remark about the U.S. Christmas bombing of Vietnam. Nixon was angered. He informed the prime minister that the Swedish ambassador scheduled to come to this country could stay home. For l7 months, the Swedish embassy was empty. Then Henry Kissinger stepped into the breach and smoothed things over. "Sweden was nothing before the Wachtmeisters arrived," said one observer of the diplomatic scene. But Sweden is something now.

This is not to say that most ambassadors do not entertain. They do, and the Swedish embassy is not regarded on the circuit as an intellectual "salon" where things happen that influence the front page from one day to the next. But one of the chief missions of a diplomat is to make sure that his nation does not wash up on the front page, like a Sovie in wht sub in the Baltic, in an unfavorable light. Ulla Wachtmeister's parties put everything Swedish in a favorable light.

"Moral qualities rule the world," wrote Emerson, "but at short distances the senses are despotic." Ulla Wachtmeister has an artist's eye because she is one, and being entertained at the Wachtmeisters is akin to eating one's way through the Louvre.

The house itself is a masterpiece, a white, Mexican-style residence set back from Nebraska Avenue near Ward Circle, its entrance protected by a sweep of manicured lawn. Eight acres of rear gardens slip down toward the baseball field at Friendship playground where a lot of American children, who didn't, until recently think of Sweden from one day to the next, play. The house appears diplomatically immune from the rest of the world. It is.

Inside all is silent, the way sun is silent as it streaks across the tapestry in the ballroom and makes the colors of one of Countess Wachtmeister's many paintings (which she executes in her spare time) spin. As always, someone is coming for dinner. The empty wine glasses, ranging down the length of a 20-foot dining room table, are full of light.

Downstairs in the basement, the second butler is at work in the laundry room. A trip through the basement is a good indication of what it takes in the way of supplies to lead the Wachtmeister's life.

There is a room for suitcases, dozens of them, another room for tablecloths (approximately 50) hung on hangars across a long rod. A third room exists for table linens, arranged by color, in cabinets. And then there is a place for one-time-use props--the centerpieces that have brought Ulla Wachtmeister fame. Some of her better ones, however, do not store.

When Sweden's minister of agriculture was in town, she saw a wild brussels sprout plant in a field outside Washington and the next thing it knew it was uprooted and sitting, against all odds that it would ever have such a splendid end, on the Wachmeister dining room table, planted in an island of fresh parsley, tastefully landscaped with spring onions and baby tomatoes. It was an edible agrarian setting for the minister of agriculture. The minister of agriculture was charmed.

One cold winter's night several years ago, Countess Wachtmeister decided to force spring. For weeks before a particular dinner party, she nursed a dozen flats of grass to life, mowing it with nail scissors to achieve proper thickness and length. The evening of her party, the dinner table was a sward of lawn, banked with flowers tied to small, potted trees. Real bird song was piped into the room, in case anyone did not get the message. The evening flew. The countess had struck again.

A Swedish balladeer came to town. Ulla Wachtmeister invited a group of women over to hear him play. "It could have been," said one guest who remembers the afternoon well, "a cold, dull recital." But the countess had other things in mind. Picnic baskets with food that might have been eaten in the 17th century were prepared for each guest. The guests strolled to the bottom of the garden and were seated on old quilts beneath some birch trees. The balladeer played and sang songs with his mandolin. All were vaguely aware that they were in the middle of Washington, but there was nothing immediately at hand that did not remind them of the simple period of time which the countess had artfully recalled.

Now if this sounds like a rather indolent life, clipping grass for dinner parties and arranging for birds to sing on cue, another harder look at Countess Wachtmeister's life must be taken. It is true that there are moments when she finds herself in improbable places, like Danny Kaye's kitchen in Beverly Hills, enjoying chow mein with King Carl XVI Gustaf of Sweden. But everybody has to be someplace, and more often than not Ulla Wachtmeister is at the Giant, pushing a shopping cart full of veal.

She meticulously shops for all the food, tends to the car when it breaks down, oversees e in wh the garden, consults with the cook, and could be seen, if one were up that early, jogging against the traffic down Nebraska Avenue in a sweat suit, with a reception for 300 looming ahead at noon. Many times, due to her husband's schedule, which whisks him unexpectedly out of town, she entertains alone.

There is an old Swedish custom called "Allemansratten" which grants every Swede the right of free access through forests or open land, regardless of who owns the property, to pick mushrooms or berries at will. Since the Wachtmeisters have been in Washington, they have entertained more guests than Sweden has lakes. The "right of access" to the Wachtmeisters' house is constant. One can take almost any day as a case in point.

October 15, 1981, 5:30-6 p.m.: A reception for 125 guests who have attended a Swedish-American industrial conference that afternoon. A brief pause. 7 p.m.: a gallery opening downtown, honoring two Swedish artists. 10 p.m.: Supper at the Wachtmeisters for 300 people after the show. Now you see them, now you don't. But always Ulla Wachtmeister manages to present a fresh face and smorgasboard. And always the butler, Mr. Johansson, conveys the concentration and mien of someone whose entire meaning in life is summed up in performing flawlessly for that one evening alone.

But it is an illusion, like the door of the embassy which opens before one has reached it. There is always someone whose assignment it is to anticipate each guest's arrival by several footfalls to spare him or her the indignity of pressing a bell.

It is a typical night. With 22 guests for dinner, the table (laid out in yellow linen with pots of red flowers on top of mirror tiles to double the bloom) is weighted slightly toward banking and tennis. There is also a newspaper columnist, several ambassadors leaving or returning to the private sector, and the current director of the FBI. Eleven black-tied columns of importance sit at the table, with their wives providing the soft, connective scrollwork in between.

The meal is served with gloved hands, in cut glass, atop gold plates, as the courses follow each other, not too little, not too much. Swedish pancakes, followed by saddle of lamb, stuffed tomatoes, and steamed snow peas. Then comes a "creation"--a standup celery look-alike which is actually upended poached cucumbers carved in stripes and filled with French mushrooms. Dessert is black currant sherbert, sprinkled with red currants that Ulla had picked at her country home in Sweden and flown back for just such a debut. The glasses are variously filled with white wine, red wine, water and a brand of Swedish aquavit that would dissolve the pebbles in Demosthenes' grateful mouth.

Down at one end of the table, the countess is listening with genuine rapt attention to the Mexican ambassador. She is neatly balanced at the other end by the count, who is attending to the words of the ambassador's wife. One turns to the right, and then turns to the left, and nobody is neglected, and perhaps not much is being said. But there are toasts--to the enduring friendship between the public and private sector, to the necessity of diplomacy "to keep us from killing each other" as the newspaper columnist said with some feeling. And there was a special, heartfelt toast from the count to the wife of an absent government official (who had been whisked away at the last moment to Europe), saying that it was very gracious and characteristic of her naturally empathetic nature for her to make the foray to the Wachtmeisters alone. Everybody agrees.

Meanwhile, Mr. Johansson is gliding silent as an iceberg behind the chairs. His eyes dart from one guest to the other without making eye-contact. Their plate, their cup, their glass, their needs: there is no actual contact between the servers and the served. And when, at the onset of dessert, Mr. Johansson bows down to the table, balancing a heavy silver tray in one hand and his remaining hand at a rightown, oversees e in wh angle at the small of his back, he is prepared, theoretically, to wait forever ... until the lady to his right sees him. Finally, she does.

Supper completed, the guests withdraw into the ball- room and reception area for coffee, liqueurs and more conversation. "If they had known what a strong man Hammarskjold was," said somebody, "they never would have elected him. They would have elected a Kurt Waldheim instead." A little ragtime music was played on the piano by one of the guests who swore he only knew Bach until finally he gave in. "Old Man River" was sung, with feeling, by the newspaper columnist. Only the Americans knew the words.

"My card," said several people to each other at the end. "My purse," said a woman, preparing to leave. Then everyone went home, protectively sheltered by umbrellas in the hands of servants, until they reached their cars.

One supposes that something diplomatically important for the future had been secured that evening. But it was difficult to know precisely what. It had, of course, been a beautiful party. And as Ben Johnson once exhaled: "How near to good is what is fair!"

Six blocks away from the Wachtmeisters' house is Janney PublicSchool. It might as well be in Stockholm.

The children at Janney and the Wachtmeisters, due to conflicting schedules, had never, until recently, had much reason to meet.

Then Ursula Cossel, a tough, demanding sixth-grade teacher had an idea. Her students ought to be more "world-minded." Here they were in Washington. The city was full of embassies. Perhaps one of the smaller ones might not let her children troop through for a few minutes to expand their point of view.

Through the D.C. public school's "Embassy Adoption" program, she called the Yugoslavian Embassy and made a luncheon date to plan an embassy event. The lunch was wonderful. So wonderful she almost forgot why she had come. Then a death in her family intervened and Cossel was forced to let the "Adopt An Embassy" program lapse in her mind.

But several Christmases ago, at the height of the diplomatic season, one of her children's parents who had met Countess Wachtmeister once volunteered to call her up and ask if the sixth grade might walk over one morning, just to see what the inside of a diplomatic residence was like.

'But of course," said the countess. "What time would be convenient for you?" The parent thought that this question should have come from her. But the countess is a diplomat, although how much of one nobody, including the countess, knew.

"I have never done this before," admitted the countess. "How many children will there be, and will it take very long?" No, assured the parent, just a few minutes, and perhaps if the embassy has any brochures about Sweden, the children could have those to take home.'

By the time the telephone call was concluded, it was arranged: 10 a.m., Dec. 18, about 45 minutes. In. Out.

To be frank, most of the children didn't know Sweden from Hungary, but for the preceding several weeks, Cossel tried to tune them up so that by the time they crossed the threshold of the Wachtmeisters' house, almost everybody knew a) where Sweden was, b) why Volvos were important to their economy, and c) that Sweden was a traditionally neutral power, whatever that means.

The children walked to the embassy. It was a cold, pre-Christmas day. They were freezing when they arrived. Then the massive oak doors opened into a different world.

Christmas carols were playing softly in the ballroom. Small white candles were lighted and blazing in every conceivable corner of the house. Holly wreaths decorated all the tables, and as the 28 children walked on tiptoe into the polished front hallway and removed their coats ("Look decent!" Cossel had ordered. All of them did.) They craned their necks around the corner into the dining room and gaped.

The dining room table had been set with linen, silver and candelabra. There was orange punch with floating banana rounds in a csees e in whrystal bowl. At least 14 different kinds of Swedish cookies and short breads, sparkled with sugar and colored candies, on the trays. And lined up, like soldiers at the children's service and pleasure, was the small but formidable Swedish staff: the cook, the second butler, and Mr. Johansson. He was beaming. Mr. Johansson is a grandfather. He looked with amusement brimming out of his pale blue eyes at the children as they tried to hold back from touching the food.

"Please," said the countess, "this is all for you. Do not be shy." After a brief moment, nobody was. There was a question and answer time. The countess thought all the questions were intelligent. She answered them all and then had another idea: "Do you want to see the kitchen?" Cossel, who had been applying faulty X-ray vision to the kitchen door, was thrilled.

Nothing was sacred. Everything was revealed--the recipes, the abalone shells she had bought for 50 cents in California, and all the pictures of the royal family that were scattered about the house.

Then, on schedule, the tour was over. Countess Wachtmeister stood at the door, looked each child gravely in the eye, shook each hand and said how much she appreciated them coming. One boy didn't want to leave.

"Would you adopt me?" he asked. For a moment the countess was confused, but then she laughed. "But I am sure," she said, brushing his cheek affectionately with her hand, "that you have very nice parents of your own." The boy shook his head.

The next day, Ursula Cossel was still bowled over. "Nothing that nice has ever happened to my children," she said. "I've never met anyone so warm. I honestly think that if I called her up and said: 'Come on over. We're having a few friends for chili,' she'd try to make it."

All the class wrote thank-you notes and the "Adopt An Embassy" program was, from every angle, a roaring success. But don't go away.

Several weeks after the tour, Ursula Cossel called up the parent who had originated the tour and said: "Are you sitting down?" The parent said yes. "Well," she continued, "you won't believe what the three shyest girls in my sixth grade did this Sunday morning." What?

"They had been at Friendship Park, near the Wachtmeisters' house playing, and they ran out of things to do. So one of them said, 'Hey, why don't we go and say 'hi' to the countess?"

"And they did?" the parent asked

"They rang the doorbell and the butler answered, and they said, 'Hi.' They just wanted to say hi to the countess, and so the butler went and got the ambassador."

According to the report, the ambassador came downstairs and took them upstairs into his bedroom to see his wife. "Oh, God," exclaimed Cossel when they told her that part. "What was she wearing?"

The countess was reading the Sunday papers, "in a sort of a pair of fancy pajamas." The count was in a robe. Hot chocolate was ordered from downstairs. Mr. Johansson brought it. The countess said how glad she was that they had stopped by. It was not, however, a full-fledged exchange of credentials. They did not, they swore to Cossel, overstay.

"Do you suppose," wondered the parent, "that now everybody else in the class will want to do the same thing?"

"I thought of that," said Ursula Cossel, "but I told the class that nobody else better try it. Once is all right," she said. "But after that it could get old."

The following week the Janney sixth grade class was riding in a bus when they passed by the Swedish embassy residence. Sweden may be a neutral power, but there was nothing neutral about the dozen windows of the bus that were suddenly thrust up while 28 pairs of hands extended themselves to wave every hand in a "thumbs-up" position.

Something important, diplomatically, for the future had been secured.