From that moment, it was clear that Bush would face a test of historic proportions - the shocking slaughter of Americans at terrorist hands. He was suddenly plunged into the first test of his presidency, one that will shape the public's perceptions of him at home and abroad.

''This is the ultimate test, the ultimate barometer. This is why we have a president,'' said Antony Blinken, for seven years a senior member of President Clinton's national security team.

Working from an emergency response plan first forged in the darkest days of the Cold War, Bush shut down the government and threw a protective shield around its top officers.

Congress and Cabinet spirited away to secure locations. Air traffic grounded. Government buildings closed.

Air Force One was en route to Washington from Florida when Bush ordered his plane to military bases in Louisiana and Nebraska before returning to the White House.

He had to show the nation and the world that America would not run from a fight, but first he had to be safe.

''You can't take anything for granted in a situation like this, certainly not the security of the president,'' said Clinton's national security adviser, Sandy Berger.

Clinton himself urged Americans to rally behind Bush, and said the new president had taken all the right first steps.

That included expressing sorrow on behalf of a stricken nation, and pledging vengeance.

''Freedom was attacked this morning,'' Bush said in the first of two sets of remarks he made before returning to Washington for a rare prime-time address. ''Make no mistake: The United States will hunt down and punish those responsible for these cowardly acts.''

U.S. officials said they suspected Osama bin Laden was the culprit behind parallel attacks in Washington and New York.

Berger said fingering those responsible should be easy, because few terrorist groups are sophisticated enough to carry out such a coordinated attack.

''This is a level of sophistication that is beyond what any intelligence outfit thought was possible,'' Berger said. ''The level of sophistication and level of presence here in the United States suggests activities that go way beyond what the FBI was aware of.''

The next step - retaliation - may be harsh. Berger suggested the U.S. responses to previous terrorist attacks will pale in comparison to the steps Bush must take.

''No single action will be sufficient once we determine who is responsible,'' Berger said. ''We'll have to buckle down for the long effort.''

Berger remembers with anguish the 1995 Oklahoma City bombing, a tragedy that struck while Clinton's political fortunes were at low tide. The Democratic president used the incident to help define himself as a leader attuned to voters' emotions.

Bush has the same dark opportunity, the same awful responsibility.

''The president's position is almost automatically transformed from a partisan political figure to a symbol of national unity, national strength and national empathy,'' said political analyst Stuart Rothenberg.

Clinton refused to talk about the political implications, saying Bush has enough on his hands.

''It's going to be difficult for him because you feel this as a human being and as a citizen, as well as a leader,'' Clinton said in a telephone interview. ''The president will handle this just fine. He'll do a good job, as long as we stick with him.''

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EDITOR'S NOTE: Ron Fournier has covered the White House and politics for The Associated Press since 1993.