Terrorism has been a fact of life elsewhere in the world for a long time, but it's getting harder and harder for Americans to remember when it couldn't _ or at least didn't _ happen here.
The pipe bomb explosion at the Atlanta Olympics is yet another reminder that the United States is especially at the mercy of groups or individuals with political causes or personal grievances.
In fact, the beginning of domestic terrorism for most Americans probably was the 1993 bombing of New York's World Trade Center, which killed six, injured more than 1,000 and virtually paralyzed neighboring Wall Street. It was underscored two years later by the bombing of the Oklahoma City federal building, with 168 killed.
Before that, terrorism against the United States focused mainly on locales outside the country _ attacks on U.S. embassies and institutions in the Middle East and Latin America, the Marine barracks bombing in Beirut in 1983, the downing of Pan Am 103 over Lockerbie, Scotland, in 1988.
The Olympics pipe bomb showed once again that terrorist acts need not be grandiose. It could be a disgruntled recluse known as the Unabomber.
"As an open society and a democracy this country is particularly vulnerable _ we have free passage, coast to coast, anyone can apply for a visa to visit, and the population is both enormous and diversified," said Philip Stern, a terrorism expert at the Fairfax Group, a New York-based corporate security firm.
Such factors provide cover for America's foreign foes.
"At the same time we have the Montana Freemen, the Branch Davidians and the people who bomb our churches _ all symptomatic of disenchantment with society among people who are willing to use violence," Stern said.
Such actions, he said, are "impossible to stop," even with stricter controls, surveillance and other security measures in confined areas such as government buildings and airports.
The size of the United States leaves it in the position of "a giant trying to swat a gnat," he said.
Pipe bomb ingredients
Investigators believe the bomb that exploded in Centennial Olympic Park was a crude pipe bomb. Here are some of the common household items used in making pipe bombs:
Clock or watch
Pipe and end caps