WWII giants -- allies but not friends; Churchill and De Gaulle, by Francois Kersaudy. New York: Atheneum. 476 pp. $ 19.95.

Ask anybody born since World War II who the greatest Englishman and Frenchman of that dramatic conflict were, and the answer is likely (and rightly) to be Winston Churchill and Charles de Gaulle.

Both men were the fearless saviors of their respective country's national honor against overwhelming odds. Since then, the world has not seen their equals. But what today's younger generation may not remember is that though both men were fighting a common enemy in Nazi Germany and both recognized in the last resort their mutual dependence, relations between them were rarely easy. In fact , they were usually tempestuous - rescued time and time again, as Francois Kersaudy makes clear, by the fact that the generous Churchill was ''a poor hater.'' (It is worth recalling, as Professor Kersaudy also records, that relations were even worse between de Gaulle and US President Roosevelt.)

Kersaudy, a historian who teaches at the Sorbonne in Paris, had the original idea of singling out the Churchill-de Gaulle wartime relationship as the theme of his book. It makes good and fascinating reading, and this Francophile reviewer has no hesitation about recommending it to Francophiles and Francophobes alike.

If there is any flaw in the book, it is that the author's singling out as his theme the personal relations of his two protagonists could leave some readers feeling that Churchill and de Gaulle were prima donnas rather than the incomparable giants they were. At the same time, in fairness to the author, it must be pointed out that his book is well documented and annotated. Not only does it explode some myths (that Churchill, for example, never said that of all the crosses he had to bear during the war, the heaviest was the Cross of Lorraine, the symbol chosen by de Gaulle for the Free or Fighting French). But Professor Kersaudy's citing of chapter and verse from records now available also reveals that both men made minor mistakes of fact in their autobiographical accounts of the war years.

Such mistakes do not distort the broad sweep of history of World War II developed by them and other writers.

That sweep, of course, involves the often complicated relationship between France and the Anglo-Saxon world, the pattern of which was set long before World War II. Both camps are on the side of liberty and human rights. Both camps fought against the Kaiser and Hitler and have had key roles in the Western alliance forged after World War II. As far back as the end of the 18th century, the American and French Revolutions were correlated at the level of human thinking in mankind's efforts to construct more enlightened societies. Yet relations between France and the Anglo-Saxon world are often far from smooth.

This is partly because of Anglo-Saxon insensitivity and partly because France occasionally finds it necessary to resort to what might be called ''spoiling'' self-assertive tactics to ensure that French cultural and political identity is preserved. The French are convinced that their culture is unique, even superior. Yet since the days of Louis XIV through those of Napoleon to our own times, they have found themselves in the frustrating position of being second to the Anglo-Saxons (first Britain, then the United States) in political influence in the Western cultural world.

In World War II, this situation manifested itself in the cruelest way. The Nazis knocked France completely out, but the British fought on under Churchill's inspired leadership, until the eventual alliance with the US ensured victory. It was only after French defeat that the remarkable de Gaulle emerged to restore to vigor and a leading role in Europe and the world a France that had been prostrate.

As he saw it from the very first day of his initially lonely crusade, this involved never allowing Britain (and later the US) to tread on French toes - even though his resources to prevent this were at first minimal.

As this book recounts in detail, long before the allies returned to the European mainland, de Gaulle sought to assert French supremacy in territories he considered French - West Africa, Somaliland, St. Pierre and Miquelon, Madagascar , and most controversially in Syria and Lebanon. The result was repeated outbursts of thunder and lightning in his dealings with Churchill (and subsequently Roosevelt) which time and again brought the two men to the brink of what seemed irrevocable rupture.

Interestingly, King George VI and Anthony Eden were sometimes more understanding of de Gaulle than was Churchill. But it was the warmhearted Churchill, outraged so often by what he thought was a lack of generosity on the part of de Gaulle, who must get most credit for preventing the lines between them from completely snapping.

Professor Kersaudy's record of all this is written in a remarkably easy and fluent English. (It is not a translation from his native French.) But his editors might have done a better job in sparing him an occasional slip - the continental ''count'' for the English title of nobility ''earl,'' for example, ''Tripolitaine'' for ''Tripolitania,'' ''Erythrea'' for ''Eritrea'' and a mistake in verbs, such as ''cannot'' for ''could not.'' Admittedly, against the author's overall achievement, these are mere molehills, not mountains.