Unlike leaders of most other countries, America's president is required under the Constitution to be both political manager and head of state. In a crisis, we ask him as political manager to explain to us what has happened and how we should respond. We look to him as the head of state for reassurance and to unite us as a people. How deftly President Bush performs in both these roles will be important to how well America withstands the struggle we are about to enter.

After foreign attacks more minor or ambiguous than last week's, presidents have had to decide whether American anger should be aroused or muffled. In 1846, after encouraging a skirmish over disputed territory near the Rio Grande, President James Polk cried that Mexico had "shed American blood upon American soil" and entered the Mexican- American War.

When the American battleship Maine was blown up off Havana in 1898 and the finger was pointed at Spain, the sitting president might well have told Americans to stay cool. Instead, in a militant fever, President McKinley took the nation into the Spanish-American War. (The Navy ruled in 1950 that the Maine had been sunk by a faulty boiler.)

By contrast, in 1915, when Germany sank the Lusitania, killing 128 Americans, Woodrow Wilson simply asked Berlin to assure him that it wouldn't happen again. (Ex-President Theodore Roosevelt, however, clamored for war.) Two years later, after German U-boats sank a number of American ships, Wilson felt compelled to take the nation into World War I.

After Americans learned of the Japanese attack on Pearl Harbor, however, there was no room for doubt. Had Franklin Roosevelt refused to ask Congress for a war declaration, he would have been impeached. He did not need to provoke Americans. Even the most vehement isolationists were demanding war. He did not need to declare a new policy. For years he had insisted that the United States arm itself and plunge into battle if attacked.

Roosevelt's greatest achievement immediately after Pearl Harbor, instead, was to make Americans feel more optimistic about winning the war than a colder appraisal of the situation would have suggested. American families were steeled when they heard that strong, self- confident voice on radio assuring them that "with confidence in our armed forces" and "the unbounding determination of our people" they would "gain the inevitable triumph, so help us God."

In October 1962, when the Soviet leader, Nikita Khrushchev, slipped nuclear-capable missiles into Cuba, President Kennedy's problem was more complex. After deflecting military demands for an immediate invasion that could spiral into full nuclear war, Kennedy had to unite Americans behind a more tentative response: blockade the island and force the Soviets to take the missiles out. He had one advantage ・he had managed to keep the presence of the missiles in Cuba a secret from the American public until the moment of his prime-time television address.

Had Kennedy been forced to cope with the Cuban missile crisis in a later age, the media would almost certainly have discovered the missiles in Cuba. Thus before the president could have spoken, angry senators and other critics might have gone on cable television to denounce the administration's incompetence in letting the Soviets sneak missiles 90 miles off the American coast. They almost certainly would have insisted that Kennedy go to the brink to fulfill his earlier pledge to keep offensive weapons out of Cuba.

Instead Kennedy had the good fortune to be able to spin the problem in the way most favorable to himself. Immediately after revealing the missiles to the public, he announced his plan for removing them ・a "quarantine" of Cuba as the first step of the American response. In that context, at a time when Americans were inclined to have faith in presidents, few viewers wondered why Kennedy was not responding by invading the island.

Kennedy's missile-crisis speech, warning Americans that they stood on the "abyss of destruction," was the most alarming address ever delivered by an American president. He used that alarm not only to convey the danger but to brace Americans for a confrontation with the Soviets that, he expected, might not end soon.

More skeptical and pessimistic by temperament than F.D.R., Kennedy did not want to exaggerate the chances for success. He closed by saying, "Our goal is not the use of might but the vindication of right. . . . God willing, that goal will be achieved." Even this modest benediction was an effort at optimism. His national security adviser, McGeorge Bundy, told me in 1988, "We sure didn't feel as good as that speech sounded."

Kennedy's response to the missiles and the way he presented it to Americans proved to be masterful. Although many of Kennedy's own mistakes had moved Khrushchev to try the Cuba gambit in the first place, he enjoyed almost unanimous national support throughout the six days until the end of the showdown. Unlike after Pearl Harbor, there was no vengeful major investigation afterward of the bungling that had allowed the crisis to happen.

When Saddam Hussein invaded Kuwait in August 1990, the first President Bush faced a decision similar to that of McKinley over the Maine and Wilson over the Lusitania. In the absence of a major direct attack on Americans, threatening war, as Mr. Bush did, was by no means an obvious response. But with his background in the oil industry and at the United Nations and the Central Intelligence Agency, Mr. Bush was particularly attuned to what Saddam Hussein's aggression in the region could mean.

The conventional wisdom of the time had it that, in the wake of our Vietnam defeat, Americans and Congress would never endorse sending half a million troops into harm's way in support of a regime most had never heard of. Nevertheless, Mr. Bush asked for and won Congressional support for his Gulf War policy, managed that war in a way that won the approval of most Americans and ended it fast enough to avoid a messy fight over the constitutionality of the War Powers Act, which demands Congressional authorization for a longer conflict.

Now we are led by President George W. Bush into a confrontation unlike any we have endured before. He must tell us what to think about a danger that few Americans were aware of even a week ago. In that respect, this crisis has historical analogies. Franklin Roosevelt had to guide Americans through the stunning realization that the Atlantic and Pacific could no longer protect us from foreign dangers. John Kennedy had to steer us through the most perilous moments of the Cold War. September 11, 2001, has shown us how vulnerable we are to terrorism. George W. Bush will have to improvise a means to fight not national armies but a new, furtive and faceless enemy.