As a teen-ager growing up in the north German port city of Lubeck half a century ago, Willy Brandt used to recall a line from Lutheran gospel that, freely translated, said "If I knew the world was going to end tomorrow, I would still plant an apple tree and pay my debts today."

There were dark forces stirring in Germany in those days. And in 1933, the 20-year-old political activist who would one day become chancellor of a postwar West Germany fled the Nazi regime and headed for Norway.

Today, there is talk of war in the air agian, talk of a U.S.-Soviet confrontation over the Persian Gulf, of catastrophe.

This talk is not foreign to Brandt, who has seen much turmoil in his lifetime. At 66, he says, the old Lutheran teaching "still best expresses my attitude" of carrying on.

Brandt is sitting, relaxed, in his suite at the Watergate Hotel. Deep circles are under his eyes but he still has the ruddy suntan he seems to maintain wherever he goes. What is missing is the big belly that used to press mightily against his vest buttons.

He is in the United States as the leader of an independent and international commission that is trying to awaken all the richer, industrialized nations of the Northern Hemisphere to the extraordinary danger that lies in the poverty, hunger and economic chaos of the world's underdeveloped nations to the south.

"There is no doubt," he says, speaking of the state of the world and his commission's work, "that things are very, very serious. And I am certainly no blue-eyed optimist. Indeed, I have become a very moderate optimist as the years have gone by.

"But I think the chances for getting through this crisis still exist, that the self-interests of the two superpowers still contain the conditions for peaceful coexistance."

Looking back over his years as chancellor between 1969 and 1974, Brandt says his greatest satisfaction grew out of his efforts to reconcile East and West, to help Germany reach out to its former enemies in what is now Communist East Europe.

Even before that, he assumed the role of peacemaker between East and West. One November night in 1956, shortly after the Russians had supressed the Hungarian revolution, Brandt arrived to calm a mob of angry Berliners threatening to invade East Berlin. He could see the Soviet tanks waiting on the other side.

Climbing onto the roof of a car near the Brandenburg Gate, he tried to explain that a clash with the Russians in Berlin would do nothing to help the Hungarians. Then he began to sing an Old German song about a comrade who had fallen in battle. The crowd began singing along with him -- and then dispersed.

Today, as Brandt moves into the latter stage of his career and more into international, rather than German, politics the next great challenge for him, he says, will be to contribute to a North-South dialogue.

There are some who believe that a new cold war will overwhelm the ability of the industrialized world to focus on this other danger to the south. There are others who believe the turmoil in places like Iran and Afghanistan will, in fact, increase attention to what the Brandt commission has been saying.

Either way, however, there is probably no one around who could argue the case for help better than Willy Brandt.

Brandt is one of contemporary history's unique characters. He is the proverbial "Good German," the man who fled Hitler and came back to help lift postwar Germany to respectability. He is the man who, as chancellor, fell to his knees at the Warsaw Ghetto memorial in Poland in 1970 in a dramatic gesture of Germany's desire for reconciliation with Eastern Europe. He is the former mayor of West Berlin who stood with President John F. Kennedy when Kennedy told all residents of that divided city that he, too, was a Berliner.

Brandt is the winner of the Nobel Peace Prize, and his name is better known internationally than that of any other German except Hitler.

Yet Brandt is also something of a tragic figure. He was embarrassed by a spy scandal in his own chancellery that forced him to resign in 1974. He is an earthy man who enjoys life. But his name pops up in gossip columns, and he shocked many Germans last year when he began proceedings to divorce his wife of 30 years, Rut, a Norwegian who was also in the anti-Nazi resistance movement in her native country.

Brandt now has another, and younger, female companion, a former assistant in his office.

In a sense, Brandt is like one of those toys with a weighted bottom that, no matter how hard it is punched, pops back up and is too attractive to resist.

Brandt his alsmo made a name for himself as a man of compassion and social justice. "One seeks influence in a democratic state," he once said, "in order to accomplish something reasonable. That alone counts."

So Brandt, while having to step down as chancellor, remains chairman of the ruling Social Democratic Party in Germany and remains an important figure in German public life, still able to turn on a crowd, excite young people or handle the intellectuals from the left better than his successor Helmut Schmidt, who is widely viewed as a far better chancellor in terms of actually getting things done.

Still, his star had faded somewhat in Germany. But it is just as bright, and perhaps even brighter, abroad. Brandt has emerged in recent years as one of the few statesmen with truly international respect and reputation.

Sixteen months ago, Brandt came to America on a mission for the newly created North-South commission. He checked into the Watergate Hotel. But he didn't feel too well. When he got back to West Germany, the pains that started here turned into a heart attack that put him out of commission for several months and may also have changed his attitude toward life somewhat.

Yesterday, back in the Watergate, he was looking far better than he has in previous years and saying that his doctors report he is now in better condition than he was before he took sick.

Brandt is a man with a sense of future, something that most German leaders either don't exhibit or don't have, perhaps for historial reasons.

At the root of this is his belief that the world could face a calamity unless it deals with the volatile aspirations of the under-developed and developing countries.

Brandt had tried to bring Soviet-bloc countries into this effort and acknowledges that the cold-war chill will make that difficult.

But Brandt says there is a growing feeling among scores of non-aligned nations of the developing world that "they are not satisfied with the Russians saying all the time that they [the Russians] are not responsible forthe consequences of colonialim. A growing number of Third World leaders say this debate should be left to historians, and that they expect the Soviets to do something. So the Russians must take care," Brandt thinks, "they cannot just take a negative mood."

In the West, Brandt says, it is crucial that a legitimate "constructive attitude" be taken toward the developing world. "There is some danger," he believe, "that Western interest might evolve only as a by-product of current tensions and global strategic concern rather than treating the problems and aspirations of these countries on their own merits."

Looking at the currently gloomy world of superpower relations, Brandt believe "it is a pity" that Soviet President Leonid Brezhnev and President Carter did not meet earlier, before relations deteriorated so badly.

Brandt is one who believes personalities play a major role in history. On the other hand, he says, there are mutual objectives -- especially control of the nuclear arms race -- that require personal responsibility for mutual survival and these objectives, he believes, will survive.

Two weeks ago, George F. Kennan, the respected historian and former U.S. ambassador to Moscow, wrote that "we are now in a danger zone. I can think of no instance in modern history where such a breakdown of political communication and such a triumph of unrestrained military suspicions as now marks the Soviet-American relations has not led, in the end, to armed conflict."

Yes, that's true, adds Brandt. But there is one crucial difference today from the past and that is the existance of nuclear weapons "which still is a pretty good guarantee, but not a complete one," that a way out of the crisis will be found.

It is still, he says, the same Lutheran phrases that make him believe that.