Memot of WWII in the Pacific; Goodbye, Darkness: A Memoir of the Pacific War, by William Manchester. Boston: Little, Brown & Co. $14.95.

For that dwindling portion of the American population (roughly, 50 years of age and over) which recalls it firsthand, World War II remains THE war. And rightly so. No Human conflict before or since -- not even the titanic and terrible struggle of the World War I -- can compare with the war of 1939-45 in scope, in breadth, in loss of life, and in its ultimate effect on global conditions.

Not the least of the reasons for this is that World War II was actually two simultaneous wars, one on the European-North African front, the other the Pacific-Asian front, the latter sweeping in a monumental 10,000-mile arc.

True, the passage of decades has tended to blur, except for those who physically participated in its battles, the memory of this Pacific war. Perhaps only its opening and closing episodes -- the attack on Pearl Harbor and the atomic destruction of Hiroshima and Nagasaki -- remain as vivid in popular thought as do such Western-front events as the Nazi gas chambers, the battle of Stalingrad, Dunkirk, D-Day, and the desert warfare. Yet, other than for the single epic struggle for Stalingrad, the fiercest, most gruesome, most difficult and mind-searing battles of the World War II occurred in the Pacific theater. Guadalcanal, Tarawa, Saipan, Iwo Jima, Okinawa, Papua, the jungle fighting in Southeast Asia, and numerous other engagements were fought under physical and military conditions far exceeding any in the West except for the winter cold on the Russian front.

In the period shortly after the end of the war there was a heavy flow of books on this Pacific war, of which perhaps "Guadalcanal Diary" made the strongest impression. Then, while the steady output of books on the European front continued, those dealing with the Pacific dwindled. While this may be understandable, given America's Europeward cultural and ethnic orientation, the courageous and costly success of the United States and its allies in the Pacific deserves to be better remembered.

William Manchester, author of "The Death of a President," "The Glory and the Dream," "American Caesar," "The Arms of Krupp," etc., helps restore and refurbish that memory in this powerful volume. Himself a several times wounded Marine Corps veteran of that conflict, Manchester returned in 1978 to the scenes of both his and others' experiences between 1942 and 1945. The book is thus written on two continually intertwined levels -- the events of those grim years on the one hand and, on the other, Manchester's soliloquizing on their significance and his judgment, mostly disappointed, on what the intervening decades have done to the areas the allies liberated from the Japanese.

This is a gripping and, on the whole, somewhat depressing book, whose somber judgments on man's folly then and now are in no wise relieved by the inclusion of a number of scatological episodes designed to amuse. Yet is it impossible for an American to read this book without pride in what his country accomplished in those days of enormous challenge, and, like Manchester, to wonder why similar energy and determination cannot be summoned to confront today's crises.