THE TRIUMPHANT success of Operation Overlord and the ensuing Normandy campaign, launched 50 years ago tomorrow, led to the destruction of German armies totaling more than 250,000 men. Its very success, however, tends to lead modern historians, in the brilliance of hindsight two generations later, to take it all for granted, as a forgone conclusion. Yet it was far from that.

Realization of the grim losses on Omaha Beach had, by mid-day on June 6, caused Gen. Omar Bradley, a competent and "unflappable" commander, to fear that his 29th and 1st Divisions had "suffered an irreversible catastrophe." He came within an inch of ordering withdrawal of the Omaha force -- representing the main bulk of the American D-Day effort.

Such a Dunkirk-style evacuation, disastrous as it would have been, illustrates just what a risky and courageous undertaking it was to invade Normandy in June 1944. It was, however, only one of the ways in which D-Day might have failed.

D-day was one of the greatest single achievements in all military history, a triumph of Anglo-American cooperation. The vast armada that set forth from England on June 6 was the largest that ever put to sea. In it were nearly 6,000 vessels of all sizes -- from vast battleships down to tiny invasion craft -- at least 11,500 aircraft, 156,115 ground troops plus three elite airborne divisions. (Of these the majority, by a margin of 10,000, were in fact British and Canadian.)

The intense risks involved in that gigantic operation have reminded me of a conversation with Gen. Norman Schwarzkopf. It was shortly after the Gulf War, and I had asked what had been his biggest headache. His immediate response: "the media."

Separately I put the same question to Sir Peter de la Billiere, the British commander in the Gulf, and received the identical answer. Schwarzkopf told me that, in the Gulf, he had gone so far as to ban all TV sets in his headquarters, lest any of his staff be influenced by what they saw on CNN's instant coverage of the battlefield.

With D-Day in mind, I asked him what might have happened if CNN had been on Omaha Beach on June 6, 1944 -- where the U.S. Army suffered some of its most severe casualties of World War II. His reply: "There would have been no D-plus-2."

In contrast to Vietnam and the Gulf, civilian populations during World War II were well shielded from the impact of battlefield carnage. U.S. censorship discreetly forbade the showing of photographs or film of any dead or badly wounded GIs. And of course there were no television cameras. History can play strange tricks; D-Day could so easily have gone terribly wrong. Secret papers recently released in London now suggest that, by 1944, it was by no means impossible for Hitler actually to have won the war -- improbable as that may seem today.

In the first place, the invasion might have taken place in 1943 -- or earlier. Stalin wanted an invasion -- Operation Sledgehammer -- as early as 1942. So did the U.S. Joint Chiefs of Staff, who were impatient with apparent British lethargy. But British caution, and -- in this instance -- good judgment, prevailed.

It was the disastrous Dieppe landing of August 1942, where the Canadians lost 3,369 out of a total force of 5,000, that illustrated the catastrophe that would almost certainly have overtaken any premature all-out invasion of northwest Europe. Success in June 1944 was predicated very largely on massive Allied superiority in the air -- which had not yet been achieved the previous year. Equally, in 1943 the British and Americans had neither the numbers of tanks nor -- more crucially -- of landing craft, that were essential to success.

Secondly, there was the weather, always particularly unpredictable in Normandy and on the English Channel. To get the right combination of tides and moon, there were only a few days in June 1944 that were acceptable. As it happened, the invasion was postponed -- on the decision of the supreme Allied commander, Gen. Dwight D. Eisenhower, from June 5 to June 6 -- because of bad weather reports. Had it been called off yet again, the next possible date would have been June 18-19. But on those days, the worst storm in 40 years struck; 800 vessels were destroyed -- together with the whole American floating harbor called Mulberry.

Thus, if postponed to June 18-19, the Anglo-American invasion force would almost certainly have suffered the same fate as the Spanish Armada in 1588 -- scattered and sunk without a shot being fired from shore, in this case by Gen. Erwin Rommel's German defenders.

Thirdly, D-Day could equally have failed if the Germans had had access to anything resembling British Ultra intelligence and were "reading our mail" as we were indeed reading theirs. In this way, Rommel would have known where we were going to land, enabling him to rush some of his 60 available divisions (including 11 powerful Panzers) to the threatened area.

An absolutely essential ingredient of Allied success on D-Day was the skillful (and British-initiated) deception scheme, Operation Fortitude. By pretending to have a whole army group under the swashbuckling U.S. Lt. Gen. George S. Patton in readiness in southeastern England, the Allies deceived the Germans into believing that the main invasion effort would take place in the heavily defended Pas de Calais. This was a logical invasion zone; the channel was narrowest here, and it offered the shortest route to Paris -- and the industrial German Ruhr. But it was also the most heavily defended.

Operation Fortitude succeeded so well that it fooled Hitler into keeping a whole German army, the 15th, tied down uselessly in the Pas de Calais even after Patton's U.S. 3rd Army had landed in Normandy, six weeks after D-Day. What might have happened had Fortitude failed is suggested by two disasters that overtook Bradley's U.S. 1st Army. During a landing exercise off Slapton Sands in April, fast German patrol boats sneaked through the Royal Navy screen to sink two LSTs. Six hundred American assault troops of the 7th Corps were killed -- more casualties than they suffered in the June 6 landing on Utah Beach. If the German patrol boats and submarines had been properly alerted by their intelligence on D-Day, losses inflicted on the Allied armada could have been devastating.

Then, when landing on deadly Omaha Beach, Bradley's men ran unexpectedly into a first-class German division, the 352nd, the only one of its standard in Normandy. Casualties were appalling, higher than anywhere else -- though slender in proportion to what was at stake, and in terms of the whole costly battle of Normandy.

If Bradley had been forced to withdraw from Omaha, and had it been repeated on the British and Canadian beaches (where, thanks chiefly to Fortitude, the landings had met only limited resistance), the cutting edge of the D-Day forces would have been lost. Almost certainly a large proportion of the indispensable invasion craft would have been lost too.

Such a reverse would have meant the almost certain postponement of another Overlord attempt to the following summer of 1945. Under the rain of Hitler's "secret weapons," the pilotless V-1 missiles (which began landing, and causing terrible damage and civilian losses, one week after D-Day), Britain's economy and morale would have been seriously impaired. With British manpower critically depleted, the main effort against Germany would have been American. The Americans, however, were under strong pressure from the "Pacific First" lobby of Adm. Ernest King, the chief of naval operations, to transfer forces -- and landing-craft -- to the Pacific.

It was Rommel's hope that, if he could destroy the Allies on the western beaches, Germany might be able to force Soviet leader Joseph Stalin into some kind of stalemate peace in the east. The 60 German divisions deployed in the west could conceivably have tilted the balance against the Red Army, which had already suffered millions of casualties. The Soviet economy was under severe strain, and as its supply lines grew longer, so proportionally did German logistical problems ease.

If D-Day had failed, at best continental Europe would have been subjected to another year -- and certainly the most terrible year -- of war before liberation. Hundreds of thousands would have succumbed to starvation. The "Final Solution" would have consumed the last remnants of European Jewry.

Finally, Hitler's scientists had been working for years on an atomic bomb. They might have been unlikely to have achieved it by 1945; but, with greater certainty, the Allies would have dropped "Fat Boy" in Europe, not Japan. With Allied ground forces stalled in the west, then in all likelihood the war would have ended with the Red Army occupying all of a "nuked" Germany, confronting the Allies with a largely communist Western Europe.

The recently released papers from the British Public Records Office show Hitler by April 1945 planning self-immolation accompanied by a terrible Wagnerian Gotterdammerung of destruction in Europe. With the war continuing through 1944 and 1945, it would have given him much greater opportunity to destroy Paris at his leisure. That none of these dread scenarios took place depended very largely on two men: Eisenhower and his ground forces commander, Gen. Sir Bernard Montgomery -- "Monty." One of Montgomery's sharpest American critics was Eisenhower's tough chief of staff, Maj. Gen. Bedell Smith, but he confessed after the war:

"I don't know if we could have done it without Monty. It was his sort of battle. Whatever they say about him, he got us there."

Almost equally indispensable, as superbly qualified to weld together harmoniously all the disparate Allied forces, was Ike. But the detailed planning, and actual command of all the invasion forces, he had entrusted to Montgomery.

As of D-Day, Montgomery was the one man on either side who could have lost the war that day. It almost certainly would have been lost, if -- in addition to the big ifs of timing, air superiority, weather and success of the Allied deception plan, Fortitude -- Monty had accepted the plans he inherited in January 1944. These had prescribed a wholly inadequate landing by three divisions. From the very beginning, Monty insisted that the Allies land five divisions on a 50-mile-wide beachhead.

As it turned out, although final victory was to be delayed another year (not least through disagreements over strategy between the Allies) success at D-Day assured the fall of Hitler. It also shaped the modern world. With U.S. predominance in the war manifestly established as her troops in Europe grew from parity with the British to a ratio of 3 to 1, D-Day was the moment when America took over lead of the alliance. Today's frontiers in Europe and the structure of the 50 years of peace that followed hark back to that success. Without it, what remained of Europe would surely have been left to face liberation by the Red Army.

Historian Alistair Horne, co-author of "The Lonely Leader; Monty 1944-45" (Harper Collins-USA, Macmillian-UK) was training in England for the Guards Armoured Division when D-Day was launched.