The attacks on the World Trade Center and Pentagon - and the responses they provoked across America - were of a scale and scope unimaginable until this week.

From ground zero in lower Manhattan, where two 110-story skyscrapers crumpled, to remote Alaska, where oil tankers stopped loading and people stood in line three hours to donate blood, the attacks shook the nation in unprecedented, unparalleled and often unexpected ways.

For the first time in history, all airplanes nationwide were grounded, from jetliners in Boston to firefighting helicopters in California. Major league baseball postponed a full schedule of regular-season games for the first time since D-Day in 1944, and it was the most games cancelled since 1918. Starbucks closed its shops nationwide. Weyerhaeuser stopped logging.

President Bush was shuttled from Florida to Louisiana to an Air Force base in Nebraska, in what is believed to be the first time a U.S. president has been flown about the country for security reasons.

''This is so big,'' said Lucas Gray, 30, a cartoon animator who lives in Olympia, Wash., and normally manages to keep the larger world at bay. ''This is the biggest, scariest thing that has ever happened in my lifetime.''

In any American's lifetime, for that matter. In lives lost, it was far bigger than the Oklahoma City bombing, which killed 168. It was bigger than any modern earthquake, flood or hurricane.

The attack drew immediate comparisons to the 1941 attack on Pearl Harbor, but even that event pales next to this week's attack. In lives lost, authorities say this week's death toll may easily outstrip that in the Pearl Harbor attack, which killed more than 2,400.

And in psychological impact, this week's attack struck deeper, wider and closer to home. The Pearl Harbor attack killed mostly military personnel, it struck at a time when the world was already girding for war, and it occurred on a remote Pacific island that wasn't even a state yet.

Tuesday's attacks, by contrast, hit twin power centers of the nation, killing mostly civilians and making Americans everywhere wonder if they might not be next in the path of violence.

''This is worse than Pearl Harbor ever was because that was our job, to be put in harm's way,'' said Bob Montgomery, 72, of Danville, Ill., who was an airman during World War II.

Part of the psychological damage incurred by this week's attack comes from its unexpectedness. The jetliners appeared literally out of the blue.

Four jetliner hijackings occurred on one day, after years without any in the United States.

Aside from Pearl Harbor, foreign attacks on American soil have been rare, and the most serious date back to the 1800s. In 1812, British forces burned the new capital at Washington, D.C., and in 1846, a brief incursion by Mexican troops across the Rio Grande started the U.S.-Mexican war, which subsequently was fought nearly all in Mexican territory.

One has to reach for long-ago accidents and natural disasters for comparisons to the numbers killed this week, in an attack that was disturbingly neither natural nor accidental.

In 1900, a hurricane swept into Galveston, Texas, and killed at least 6,000 people. In 1889, the collapse of the South Fork Dam in Johnstown, Pa., killed more than 2,200. An 1865 boiler explosion on a Mississippi River steamboat killed 1,547.

In any other context, a hijacked jetliner crashing into the Pennyslvania countryside would be one of the biggest stories of the year. This week, it is merely a sidebar. When a 47-story skyscraper in the World Trade Center complex collapsed, it was barely mentioned amid the greater cataclysm.

There may be unprecedented measures still to come. NATO allies declared that if the attacks turn out to have come from abroad, as suspected, then the United States could make use of NATO's one-for-all and all-for-one principle - in place since 1949 but never invoked.

It all leaves Americans searching for a way to map this week's terror on a scale that makes any sense at all.

''It's like a replay times tenfold of what we've been through here in Oklahoma City,'' said Dr. R. Murali Krishna, a psychiatrist. Even there, he said, people are wondering: ''How can this happen, how can the most powerful nation in the world be attacked by somebody?''