IT IS NOW 40 years since 4:30 a.m. on Sept. 1, 1939, when German troops stood to arms as zero hour approached for the invasion of Poland.

The governments and foreign diplomats in London and Paris, Berlin and Rome continued their negotiations even as the first shots were fired. The Allies remained optimistic. Perhaps some combination of carrot and stick could persuade Hitler to halt, to reconsider. Had he not, after all, suffered all through World War I? What hopes could he have against the prestigious French Army, the Maginot Line, the Royal Navy and Air Force, the immense resources on which Britain and France could draw? Surely it was senseless to risk so much over a few bits of Polish territory?

In Berlin the assumptions were different, the optimism much the same. Surely Britain and France would back down, as they had when the Rhineland was reoccupied, Austria annexed, Czechoslovakia dismembered. Did those two soft, cowardly, disunited nations really care a whit about Poland?

But Sept. 3 saw Britain and France declare war. A British ultimatum reached the Reich's Chancellery early that Sunday morning: Withdraw from Poland or face the consequences. It stirred forebodings, doubts, but no retreat. Hitler sat silently as it was read aloud. Finally he turned to Foreign Minister Joachim von Ribbentrop, a self-proclaimed expert on Britain, and rasped out, "What now?" Ribbentrop had little to say. Hermann Goering alone burst forth, "If we lose this war, then God have mercy on us!"

The mood in London was no less tense. Air raids were expected, massive bombardments that would destroy lives, property and national morale as well. Nothing happened, and the barrage balloons swayed innocently in the breeze as Prime Minister Neville Chamberlain addressed the House of Commons. "This is a sad day for all of us, and to none is it sadder, for everything that I have believed in during my public life has crashed into ruins." A remarkably honest admission, but was this a call to arms?

For that another leader was needed, and his presence already was noticed. Churchill's appointment that day as first lord of the Admiralty, which he had quit in 1915, sent a buoyant signal through the fleet: "Winston is back."

So began World War II. It was confined initially to Europe and the North Atlantic, but soon engulfed the Mediterranean, the Balkans, north and east Africa. By 1941 it had become the first (and perhaps the last) global war, a war of all the seas, the oceans and the continents, a war of peoples rather than of armies only, a war whose sheer immensity dwarfed anything that had occurred since the dawn of history.

Neither the Reformation nor the French Revolution, neither the rise of Christianity nor that of Islam, neither the most virulent epidemic nor the most destructive catastrophe of nature, ever imposed itself so intensely on so many people in so much of the world. For millions it always will be the war, an episode in their lives whose importance no other historical event can rival.

For it was a total war, the line between civilian and soldier being virtually erased as governments sought to mobilize all their nation's energies for combat, for production, for all the tasks of war. In this the anti-Axis coalition, particularly Britain and Russia, were vastly more efficient, more practical and systematic than the Axis powers, which permitted far more consumer goods and other frills than listeners to their militaristic rhetoric ever would have expected. The line disappeared entirely for civilians in the resistance movements, the guerrillas, saboteurs, propagandists who fought the Axis occupation forces and collaborationists in scattered combats of intense savagery.

It was a war of unparalleled death and destruction, ferocity and sheer terror, much of it inflicted on civilians who, increasingly, were treated as perfectly fair game. Of the 40 or 50 million dead -- some 20 million Russians, nearly 6 million Jews, perhaps 4 million Chinese, 4.5 million Germans, about 2 million Japanese, several millions Poles and Yugoslavs -- the list is interminable -- at least half were civilians.

Here was a terrible forerunner of later massacres in Algeria, Vietnam and elsewhere in the Third World. To be a civilian in Britain, Germany or Japan as bombing reached its peak, or to struggle for survival in German-occupied Poland and Russia, Greece and Yugoslavia, was to stand virtually in the front lines. And to be a Jew in Hitler's Europe was to live more closely with death than any combatant except the Japanese kamikazes. Nevertheless, civilian morale held fast, women faced air raids, hungry children and lines at the grocer's as well as men faced battle, and there was none of the serious domestic unrest known in World War I.

It is important to note that Americans experienced little of this anguish, suffering some 400,000 dead but no destruction whatever while enjoying an economic boom that all the world envied.

It was a war of genocide and racism generally, and this underlay much of the civilian death toll. Genocide is as old as history, and there were ominous hints for the future in the massacre of Armenians by Turks in World War I, and by Greeks and Turks of each other in their war that followed. But an intensive policy of genocide, ordained at the top and carried out comprehensively by an elaborate bureaucracy, was another matter.

Nor was it the Jews alone who suffered, all too often to the satisfaction of other Europeans. Goering spoke casually, late in 1941, of the starving Russian prisoners of war who, "having eaten everything possible, including the soles of their boots, have begun to eat each other, and, what is more serious, have also eaten a German sentry." And Heinrich Himmler was delighted by the opportunity to destroy Warsaw, the very heart and symbol of Polish nationalism, when the Poles revolted in August 1944; nearly 250,000 Poles were killed, and the SS systematically demolished the inner city, soldiers with flamethrowers spraying building after building like conscientious gardeners hosing their flower beds.

It was a war of imperialism by the Axis powers, an attempt to divide up the world for their own benefit. Italy fought solely to seize the Mediterranean and the western Balkans, while Japan's rhetoric about "The Greater East Asia Co-Prosperity Sphere" barely disguised its ambitions in China, Southeast Asia and the western Pacific. Hitler's henchmen squabbled about control of an empire of slaves and raw materials in the Russian steppes, of vast estates and military garrisons where the SS would stand watch, remaining lean and hard while preserving the Thousand-Year Reich.

For the Russians, Hitler sketched a grim future. "We shall give the natives all they need: plenty to eat and rotgut booze. If they don't work, they'll go to a camp and they'll be deprived of alcohol." Italian and Japanese colonialists were less crude, but equally harsh about retaining full control. The replacement of Western colonialism with an Axis version would have been disastrous for the subject peoples, most notably for the Gandhis and Nehrus -- how long would they have survived? -- whose struggle for independence was soon to bear fruit.

It was a war of ideology, a struggle in which liberal democracy confounded those oracles of the extreme Right and Left who long had been predicting victory for a totalitarian wave of the future. Appeasement, defeatism, isolationism were cast aside as the British fought for national survival, the Americans for security and a better world, the Free French and the other resistance movements for honor, patriotism, life itself.

There were, obviously, less idealistic war aims as well, those of power and spheres of influence, economic expansion and the restoration -- a vain hope, as Roosevelt understood -- of colonialism. But it is dangerously mistaken to rank these aims at the top, as have some recent revisionist historians, and to treat wartime idealism as a mere cover-up for alleged State Department and Wall Street machinations.

Roosevelt and Churchill demonstrated that governments having solid popular support could operate fairly and relatively openly even in wartime; that civil liberties and legislative processes could be preserved; that civilian leaders could make strategy without the continual bickering with the military that had marked World War I; and that (aside from the infamous maltreatment of the Japanese-Americans) there need not be the racist and political hysteria of that war. In effect, they showed that unified democracies could wage war successfully without ceasing to be democratic.

It was a war of science and technology, the first electronic year, in which even the airwaves and the stratosphere became battlegrounds, while the lowliest soldier might be found using the most sophisticated weapons. The science fiction of yesterday became the reality of today, and scientists themselves entered the corridors of power, in the West as the problems of bombing, aerial defense and anti-submarine warfare became crucial, in Germany as interest focused on weapons that could turn the tide: rockets, jet airplanes, snorkel submarines.

The scientist no longer was merely a specialist hired to solve particular problems and create weapons to specification. Now he participated increasingly -- and from the outset -- in decision making, influencing its direction and results, its procedures and personnel. And with the atomic bomb, the scientists achieved a revolution in warfare that ensured their presence in the highest strategic councils of the future.

It was a war of shifting power relationships, a war creating remarkable transformations in international politics. "The German problem," which had bedeviled Europe since 1870, was no more. Germany was defeated, its territory diminished, its provinces occupied, its appetite for world power checked, perhaps eliminated. Economic growth, not military might, has been its direction since 1945. So it has been for Japan, which lost -- its only major defeat in history -- an empire and the denomination of East Asia, but since has gained a flourishing domain of international trade, investment and finance. Britain and France emerged from the war greatly weakened monetarily and politically, their population and economic resources clearly insufficient for great power status, their colonies pressing for an independence that could no longer be denied, a freedom which Britain granted gracefully, France only after wars in Indochina and Algeria.

It was to the United States and to Russia that power had gravitated. Both were able to overcome serious defeats in 1941-42, to generate enormous military and economic strength, to project that strength over vast distances, to maintain cohesion both at home and in an alliance fraught with ideological rivalries. Each country filled a unique wartime role, the Russians by "ripping the guts out of the German army" (the phrase is Churchill's); and the Americans by intervening with men, or war materials, or both, in virtually every Allied campaign from 1942 onward.

It was a war of engines, hence of speed, maneuver and surprise, where striking hard was linked to striking fast and where powerful forces might pounce from an empty sky or emerge from an open sea. The ability to mass overwhelming firepower, plus the opportunity for maneuvering under fire provided by the tank, the airplane and the carrier task force gave the offensive a scope it had lost when artillery and machine guns strangled movement on the battlefields of World War I.

The new resources that technology provided were exploited by many commanders of solid ability, some even of brilliance. The Neanderthal brass hats of the trenches gave way to leaders like Mannstein and Rommel, Patton and MacArthur, Zhukov and Konev, and many others. There were, of course, failures, particularly early in the war: the French in 1940, the Italians in Greece and Libya, the Russians in mid-1941, the Americans at Pearl Harbor, the British in Malaya. But even these defeats contained little of the grinding slaughter of trench warfare. For officers had learned that surprise, flexibility, good morale and close cooperation with allies and other services counted for more than doctrinaire planning and excessive reliance on superior strength.

So the sullen, hate-filled masses of World War I gave way to armies that displayed amazing resilience and cohesion. The Germans, the Russians, the Japanese in particular, accepted terrible casualties without breaking; the British Tommy remained cheerful even during the defeats of 1940-42; and the Americans surprised outsiders by battling aggressively in the assault and whenever the chips were down. The staunch human qualities all sides demonstrated under fire belied those prophets of doom who associated another world war with the downfall of civilization, an ugly weakening of moral fiber and a rapid decline into revolution and anarchy. It was not to be. The human race was tougher and more traditional than that, more ready to struggle for survival by clinging to basic hopes and beliefs.

It was a war without exaltation or grandeur. It is tempting, particularly for Americans, to regard the war fondly, as an era of national purpose and moral confidence, to lend it the glamor of heroism and crusading zeal, and to be fascinated by this or that brave exploit against great odds. All the more reason to heed the Norwegian resistance fighter who, having seen much death and danger at close range, expressed himself with immense realism:

"Though war can bring adventures which stir the heart, the true nature of war is composed of innumerable personal tragedies, of grief, waste and sacrifice, wholly evil and not redeemed by glory."