Dunkirk: telling the whole story; Dunkirk: A Patriotic Myth, by Nicholas Harman. New York: Simon and Schuster. $ 12.95.

If the Great War is now viewed through the filter of history as an agony of folly, World War II may still be seen as the "war of necessity." Because of Hitler's monomaniacal cruelty, the struggle seems a clear victory for the Children of Light over the Forces of Darkness, Europe saved and tyranny defeated. We are still stirred by the defiant singing of the Marseillaise in Casablanca, by the rugged heroism of the Royal Air Force during the Battle of Britain, by the memory of our own doughboys flooding the beaches of Sicily and Normandy.

But like many great stories this rendering contains the danger of oversimplification; the myth obscures the reality. Marcel Ophuls has resurrected the sorrow and pity of France and the betrayal of the Vichy government. Americans should be reminded that they did not exactly rush off to save a beleaguered Europe; Pearl Harbor was a year and a half after the fall of France. And now Nicholas Harman, an English journalist, has written a crisp, no-nonsense documetary history to point out that Dunkirk (Winston Churchill not withstanding) was not quite Britain's finest hour.

This is revisionist history at its best, minus axes to grind. Harman details the posturing of British and French leaders as Hitler, fresh from victories over Poland and Czechoslovakia, turned to crush Europe in the iron jaws of his vaunted blitzkrieg.The allies, as Harman sees it, were quite prepared to fight World War I and expected another war of attrition which, however terrible, would allow them time to marshal their defenses -- time which the German armies in their lightning strikes through the Low Countries were unwilling to grant.

Thus, as the German forces circumvented the Maginot Line and struck, the stage was set for the fall of France and bitter days of Dunkirk. What Harman reveals is a tangle of jealous suspicion at the high command, an Allied leadership in disarray, and a constant sense of betrayal that ended in a separate peace for Belgium and for France.

The evacuation itself took place from May 27 to June 4, 1940, and it is not a pretty story. The figures reveal a curious disrepancy: Of the 310,000 troops evacuated, almost 190,000 were British, most of whom arrived home in the early days of the evacuation. So embarrassing were the early numbers that Churchill, responding to French threats to break off the alliance, ordered a one-to-one evacuation of British and French troops on June 1.The French were essentially correct in asserting that they provided rear guard support for the fleeing British; only French troops (approximately 60,000) were present to surrender to the Germans who finally broke through on June 4. Furthermore, as the French charged, the RAF was being withheld for the protection of Britain. So the French complaint that they were extended a certain double dealing at their lowest moment has a certian validity.

But Harman refrains from easy scorn. He agrees with the assessment of the British leaders that the battle of France could not be won and that, ugly and perhaps shameful as it was, British forces did have to escape to fight another day. Yet that fact could not be advertised.

Perhaps the most interesting chapter of the Dunkirk affair is Harman's revelation of the manipulatin of the press back home. Only after most British troops had returned did the War Office allow the news of the evacuation to leak out to the British public. Also, though Churchill evoked an image of a flotilla of civilian vessels braving the Luftwaffe and German guns to arouse the British people, in fact less than 10 percent of the troops were actually rescued by civilian boats and many of those were commandeered by military officers. But the patriotic myth, as Harman calls it, was born.

It was a useful myth; and, to be sure, the story of those terrible days at Dunkirk included much bravery, British as well as French, so that, in retrospect , the treachery seems hapless rather than deliberate. We may understand how during the dark days to come the truth was too dispiriting to be revealed. Only now with the happy ending of Allied victory and the luxury of 40 years' perspective may we look back with dispassionate eyes and perhaps draw a lesson.

n a Europe united through NATO and the Common Market, the forcesof jealous nationalism are still jockeying for power and special interests, Dunkirk -- with all its everberations -- should serve as warning against a small-minded selfishness; and in telling that story honestly, Nicholas Harman has performed a valuable service.