WASHINGTON (AP) _ Even the Americans, who did it, didn't think it could be done. With grit and guts, they carried off the greatest humanitarian air rescue in history, an unambiguous triumph over Josef Stalin at the cruel start of the Cold War.

Even now, 50 years later, it seems impossible that 2 million West Berliners could be kept alive by air, provided food and coal, shoes and medicine, everything, by planes.

From the sky came salvation. The Berlin airlift: More than 2 million tons of supplies delivered by more than a quarter of a million flights over 285 days, planes landing once every three minutes.

The airlift's 50th anniversary will be celebrated throughout this year in Germany, starting with a visit on May 13 by President Clinton to Berlin. He will recognize the steadiness of Harry Truman, the skilled, bone-wearying flying of American, British and French airmen and the perseverance of the Berliners.

On June 24, 1948, with 30 powerful divisions at their disposal in East Germany, the Russians cut off all access to West Berlin, by rail, road or waterway. They wanted to force the Western allies to withdraw from the city.

Instead, as an expedient, without asking anyone, Gen. Lucius Clay, the American commander, ordered an air supply effort. Planes flying over communist terrain lumbered into Berlin, night and day, in all weather.

Planes much smaller than today's behemoths carried 2,326,406 tons of cargo _ tons, not pounds _ to the 2 million people in the American, British and French sectors.

They flew 278,228 flights _ an average of 599 a day, 25 an hour.

At peak periods, a plane landed once every three minutes at each of the three airlift airfields _ Tempelhof, Gatow and, later, Tegel, built by the Berliners themselves from the rubble of war.

Amazed Berliners gathered in clusters to watch the spectacle, which lasted 15 months.

The airmen called it ``Operation Vittles.''

``We were on the ground for about 15 minutes,'' recalls pilot Guy Dunn, 75, then the 25-year-old son of an auto dealer in Bland, Va. ``Then you had an hour back at the on-load station, then you flew another trip to Berlin _ four flights in 12 hours, then you were off for 12 hours, then you were back flying.''

During his airlift tour, Dunn, now retired in Fort Walton Beach, Fla., flew 790 hours, covering about 142,000 miles in short hops between West German airfields and Berlin.

Once in a while, he says, Soviet fighters harassed the American planes, ``but they never shot at us.'' To underscore their resolve, the Americans had landed a fleet of B-29 bombers in England, capable of carrying the atom bomb.

As late as October 1948, with winter coming _ and no city in Europe has worse winters for flying _ there were still doubters among the Americans. Better to abandon Berlin to the communists than see it starve, they counseled.

``It was one of the worst, wettest winters in Europe in years,'' says former Lt. Dunn. ``The temperature was near freezing most of the time, so you had to contend with ice on the ground and in flight. When the weather was real poor, you were flying at rooftop level.''

As the airlift got organized, displaced persons, refugees of the war, were brought in to unload the planes.

For a time the planes flew in five flight levels, each 500 feet apart, and in each level they flew 15 minutes apart. A plane that could not land had to return to its home base before trying again.

It was not without costs: Through accidents, the Americans lost 31 lives. Some planes overshot the runways; some crashed into each other.

Ground crews in Frankfurt, 275 miles away, worked around the clock to service the two-engine American C-47s, the military version of the DC-3, and later the larger, four-engine C-54s, comparable to the DC-4.

``During the war, I used to run into my buddies in fighter units,'' recalls retired Brig. Gen. Sterling Bettinger, 82, of Colorado Springs, Colo., the chief airlift pilot. ``They'd say, 'You guys are allergic to combat.' We weren't too highly respected in the air transport business. But after the Berlin airlift, we gained a lot of respect. Everybody realized that the airlift preserved the country.''

When the Russians slapped on the blockade, Gen. Clay knew exactly what he wanted to do. He wanted to slap back.

Clay wanted to send an armed convoy down the Autobahn to shoot its way through to Berlin.

But Truman had a different idea. Or two ideas, seemingly contradictory. He didn't want to abandon Berlin. And he wouldn't risk a war.

``We are going to stay,'' Truman told his Pentagon commanders. ``Period.''

But he did not say how.

Clay improvised, with no planning for such a massive undertaking and with little optimism that it would work. Truman endorsed it, and made it American policy.

The crisis had been sown in the closing weeks of World War II when the allies agreed to let the Red Army take Berlin. With that, the old German capital, 110 miles inside the Soviet occupation zone, was divided into four sectors: East Berlin controlled by the Russians and West Berlin by the British, French and Americans, a free enclave in a sea of communism.

By 1948, the postwar world well knew that an ideological state of war existed, between the communist East and the free West.

In February 1948, the communists took over Czechoslovakia. Later that spring, the Russians blockaded Berlin briefly, then backed off.

So when the Russians again sealed access to the city in June, ``People thought, 'Here we go again. The Russians are tweaking our noses,''' says Robert van der Linden, a historian at the Smithsonian Air and Space Museum.

But this time, the Russian purpose was not to harass the Western powers but to force them out.

The airlift was small potatoes at first: A scratch force of C-47 cargo planes flew in 80 tons on the first day.

Clay estimated that in ``a very big operation'' 700 tons could be ferried in per day _ pathetically short of the 4,500 tons of supplies that would be necessary at minimum.

Holding thumb and forefinger apart, Clay said at the time, ``I wouldn't give you that for our chances.''

In retrospect, it was ``a rather desperate expedient,'' says diplomatic historian John Gaddis of Yale. Truman wrote in his memoirs: ``Even the Air Force chiefs themselves at first had serious doubts it could be done.''

The doubters were proven wrong. Stalin, the Soviet dictator, had played into American strength _ the ability to move mass amounts of cargo.

He should have known: During the war, the Americans had flown tons and tons of war materiel to Russia, sustaining the Red Army while it drove the Nazis out.

And, in Asia, American planes had demonstrated their delivery capabilities by flying ``the Hump'' over the Himalayas to supply the Chinese nationalists in their fight against the Japanese _ ``the most hazardous flying in the world,'' according to Air Force historian Roger Miller.

So it was only natural for the Americans to send for Maj. Gen. William Tunner, who had orchestrated the ``Hump'' operation.

``General Tunner transformed this chaotic improvisation into a model of efficiency,'' says historian Douglas Brinkley. ``His goal was to reduce the airlift to a steady, even rhythm with hundreds of planes doing the same thing every hour of every day. It became a perfectly learned routine.''

Brinkley says three factors made the airlift a success: ``Truman's ability not to overreact; the Berliners, who had the fiber to tough it out, and Tunner.''

On May 12, 1949, the Russians threw in the towel and lifted the blockade. The airlift continued through September, to make sure Berlin had ample supplies.

The Russians suffered a propaganda loss that scuttled chances of the strong Communist parties of Italy and France taking over those countries through the ballot box.

``It was clearly a confrontation and we clearly won it,'' says van der Linden. ``It rates up there with the Cuban missile crisis.''

Berlin remained a flashpoint until 1989, when the wall erected in 1961 to keep East Berliners from fleeing was torn down.

But the airlift gave impetus to the creation of NATO and the establishment of West Germany as an independent nation. It cemented America's commitment to the defense of Europe.

``It changed the American public's perception of the Germans,'' says van der Linden. ``They no longer were Nazis, they were allies. And the Americans were no longer conquerors, but friends.''