President Bush, admitting a "quiet, unyielding anger" over the thousands killed by terrorist attacks on the Pentagon and the World Trade Center, addressed the nation from the Oval Office last night vowing justice and revenge.

"These acts of mass murder were intended to frighten our nation into chaos and retreat, but they have failed," the president said. "Our country is strong. A great people has been moved to defend a great nation."

Bush said the federal government and financial markets would be open for business today, and he said Americans would unite against its enemies and those governments that harbor them. "None of us," he said, "will ever forget this day."

The address from the inner sanctum of the White House capped a day for the president and the country's national security apparatus unlike any before it. Bush, after hearing of the attacks while visiting a school in Florida, boarded Air Force One and, escorted by fighter jets, hopscotched to military installations in Louisiana and Nebraska before returning to Washington.

Earlier in the day, national security officials feared the country might be under a full-scale attack, and unfounded reports of an attack on Camp David and the State Department caused the Secret Service to believe the president was being targeted. At one point, officials even contemplated keeping the president and his entourage underground overnight at the Strategic Command near Omaha, at the controls of the country's nuclear arsenal.

In what was unquestionably the greatest test of the Bush presidency, the administration stuck to a game plan that has guided it through the first eight months of the presidency and, before that, his campaign: understated and controlled. For much of the day, Bush made minimal appearances in public, teleconferencing with his national security team. His Cabinet and top aides, struggling to get a full understanding of the attack and to coordinate a response, provided scant public information.

While a senior aide said in the morning that Bush was eager to get back to the White House to convey a sense of stability, the president spent most of the day on the move, seeking secure locations -- at one point riding in an armored, camouflaged Humvee under the guard of soldiers in fatigues toting machine guns.

At Offutt Air Force Base, home of the U.S. Strategic Command, Bush held a 65-minute meeting of his national security team, including Vice President Cheney and national security adviser Condoleezza Rice, who were hunkered down underground in the White House situation room, and Defense Secretary Donald H. Rumsfeld, who strategized with the military chiefs of staff in a Pentagon still on fire after being struck by a terrorist-hijacked jet. After returning to the White House last night and delivering his speech, Bush met into the night with security aides.

Word of the tragedy first came to the president in the hallway of a school in Sarasota, Fla., moments after the first plane hit New York's World Trade Center. He went to a private room, where he spoke by phone with Rice; it appeared then that the matter could be just a terrible accident. Then, at 9:04 a.m., while Bush met with second-graders, staff chief Andrew H. Card Jr. whispered in his ear that a second plane had struck. Bush's sunny countenance went grim.

After Card's whisper, Bush looked distracted and somber but continued to listen to the second-graders read and soon was smiling again. He joked that they read so well, they must be sixth-graders.

After huddling with advisers, Bush entered the school's media center for what was to have been an education speech. He looked stunned, but by the time he reached the podium, he was composed and at 9:30 a.m. delivered the chilling news of "an apparent terrorist attack on our country."

The mood onboard Air Force One, which left Sarasota at 9:55 a.m., was tense. Aides, stewards and Secret Service agents were not told where they were going.

A fighter jet could be seen off the right wing. The plane climbed well above 40,000 feet, an apparent security measure, and those aboard were told not to use their mobile phones as the plane descended because the signals could disclose the plane's location. Bush, viewing television footage of the devastation, spoke with Rumsfeld and twice with Cheney, and he gave authorization for the military to raise its war readiness status to DefCon 3, with DefCon 5 being war.

When Bush landed, at Barksdale Air Force Base in Louisiana, the plane was surrounded by Air Force personnel in full combat gear -- green fatigues, flak jackets, helmets, drawn M-16s. Security advisers sifted through the reports of attacks -- including the crash near Pittsburgh and an unfounded account of a car bomb at the State Department -- to determine which were real. Bush, borrowing a general's quarters on the base, spoke twice more with Cheney and again with Rumsfeld. He also took a call from Sen. Charles Schumer (D-N.Y.) and would later return calls from New York Mayor Rudolph W. Giuliani and Gov. George Pataki on his flight to Nebraska. Bush did not speak to other members of Congress, his spokesman said, directing Cheney and aides to do that.

At Barksdale, Bush elaborated on his earlier remarks in a second address that was quickly broadcast. "The resolve of our great nation is being tested," Bush said. "But make no mistake, we will show the world that we will pass this test. God bless."

Back at the White House, officials working in the West Wing and the adjacent Eisenhower Executive Office Building heard a droning horn at about 9:30 a.m. followed by a standard fire alarm recording ordering an evacuation. Uniformed Secret Service agents holding machine guns stood on the corner of Connecticut Avenue and H Street, guarding the street leading to the executive mansion where staffers park their cars. About 40 senior aides set up temporary camp in a nearby office building. Finding mobile-phone networks overloaded, "most folks went home," said a White House aide.

First lady Laura Bush, on Capitol Hill to testify on education before a Senate committee, was ushered by motorcade to a secure location outside the White House with her aides. The first lady spoke by telephone with her husband and her daughters, who just began their second year of college.

Transportation Secretary Norman Y. Mineta, overseeing the first grounding of all commercial air traffic in the nation's history, joined senior Bush staff in the White House command center. The senior official in the White House through the day was the vice president, joined by his staff chief, I. Lewis "Scooter" Libby.

Bush, who left most of his press corps behind in Sarasota, left others at Barksdale when he left at 1:37 p.m. Those on the flight said arrangements were being made for the party to stay in secure facilities underground at Strategic Command. But after security aides determined that it was safe for Bush to return to Washington, Air Force One flew to Andrews Air Force Base, and the president stepped off Marine One on the White House lawn at 6:58 p.m. for his 8:30 speech.

The speech, written by counselor Karen Hughes and speechwriter Michael Gerson, combined a warning of reprisal with a reading of part of the 23rd Psalm. Elsewhere in the White House, where the flags were at half staff, aides had returned and were redrawing Bush's schedule to reflect the nation's altered circumstance.

Allen reported from Sarasota. President Bush and Chief of Staff Andrew H. Card Jr. talk aboard Air Force One on the way to U.S. Strategic Command headquarters at Offutt Air Force Base in Nebraska.