This nation needs a spacious attic, and one corner of it is in Suitland, Md. Some corner: 25 buildings house the Smithsonian's reserve collection of aircraft. A Soviet anti-aircraft missile sits next to John Kennedy's campaign plane, "Caroline." There is a Grumman "Avenger" of the sort the young "George Bush was shot down in. But the sight that causes a visitor to catch his breath is the dull silver fuselage of a B29 bearing the stenciled name "Enola Gay."

The atomic age, which began in secret in a New Mexico desert at dawn 40 years ago July 16, announced itself 21 days later when the Enola Gay's bomb bay opened. The fuse -- the lense David Greenglass had sketched for the Rosenbergs' spy ring -- unleashed neutrons that created in 22 pounds of uranium an explosion that occurred in one-tenth of a millionth of a second.

The flight of the Enola Gay began, in a sense, in 1932 in Cambridge, England, in Cavendish Laboratory, when James Chadwick discovered the neutron, the key to penetrating the atom's nucleus and unlocking energy from matter. Thirteen years later, when the B29 fliers asked what they had volunteered for, they were told their 509th Composite Group was "going to do something different."

When they reached Tinian, in range of Japan, Tokyo Rose was on the radio reading the doggerel that Americans on Tinian had written to ridicule the 509th's strange training mission:

"But take it from one who knows the score, the 509th is winning the war."

Well, yes. At the stunning moment in New Mexico, when Robert Oppenheimer had thought of "the shatterer of worlds," a general simply said: "The war's over."

As the Enola Gay approached Japan, the copilot was writing a letter to his parents. He wrote this sentence: "There will be a short intermission while we bomb our target." Next, he wrote this in a wild hand: "My God."

The government committee that had kept the secret of the bomb project (neither Adm. Nimitz nor Gen. MacArthur knew about the bomb until July) said it should be considered not just as a weapon but "in terms of a new relationship to the universe." Forty years on, it would be extravagant to say the new technology of mass destruction has had such a transforming effect, spiritually or practically. Why should it have? Conventional munitions on the ground at Verdun killed many more people than nuclear weapons have. The same was true at the Somme, 17 years before the neutron was discovered.

Pug Henry, protagonist in Herman Wouk's "War and Remembrance," says: "Either war is finished, or we are." It is too soon to say whether we are, but war certainly is not. It flourishes beneath the nuclear umbrella. However, the first two bombs were war-enders and life-savers. They prevented perhaps a million American casualties and probably spared Japan at least 10 times the 210,000 deaths they caused.

Each bomb killed fewer people than were devoured in each of two B29 raids on Tokyo.

Those raids were previews of what the autumn would have brought, but for the bomb. Japan had 2.3 million regular army soldiers, 250,000 garrison troops, 5,000 kamikaze aircraft. Children were being trained to strap themselves with explosives and roll under tanks. There were potentially 30 million partisans with the will to die shown by Japanese soliders on Okinawa and Iwo Jima.

U.S. officials were too uncertain of the new technology to risk a nonlethal demonstration for Japan that might have been a dud, producing hardened Japanese resolve. There were just two bombs. Until after the second bomb fell, Japan's regime remained resolved to have a face-saving (and perhaps compromise-achieving) blood bath.

The use of the bombs was seized upon by persons eager to portray America as a crude giant whose technological power is disproportionate to its moral maturity, a nation with a cold Machiavellian heart beating slowly beneath a thin lacquer of idealism. But Machiavelli's bad reputation is the unjust price he paid for being an unsentimental moralist in a world addicted to moral evasions.

He said that a material and mental capacity for violence underlies a great nation's power. The moral imperative is to economize violence by distinguishing between legitimate and illegitimate uses. Legitimate uses are to reduce violence and preserve or promote good objectives.

In a few years, the Enola Gay is to be displayed with other aircraft at a new museum at Dulles Airport. It will be visited by hundreds of thousands of fathers and their children and grandchildren who would not be alive had the two bombs not made unnecessary an invasion of Japan. The museum will be a school teaching sobriety, where Americans can ponder the Enola Gay's role in a deed profoundly Machiavellian and moral.