ASHINGTON, Sept. 11 — For a quarter-century now, terrorism has been a constant concern all around the world, but in general it is Americans at home and abroad who have been the targets of the largest and most destructive attacks.

Most of history's deadliest terrorist incidents were directed at American civilians, including the bombing of Pan Am flight 103 over Lockerbie, Scotland, in 1988; the bombing of a federal office building in Oklahoma City in 1995, and the nearly simultaneous explosion of car bombs outside United States Embassies in Kenya and Tanzania in 1998.

Terrorism experts see today's coordinated attacks, at the World Trade Center and the Pentagon, as the culmination of a 20-year trend toward assaults that aim to kill many people in technically complex operations. Often they are orchestrated by assailants who rarely take responsibility or state clear goals.

"We have seen the number of terrorist incidents go down, while the number of casualties soars," said L. Paul Bremer III, a former Reagan administration terrorist expert who helped draft a national counterterrorism strategy released last year. "The trend has been toward revenge attacks where terrorists seek to maximize the number of innocent people killed."

Not only Americans suffer horrific deaths in such attacks. The State Department estimates that India suffered the largest number of terrorist incidents in 1999, for example. Indians were also the victims in the terrorist attack that caused the the highest death toll before today's: the bombing in 1985 of an Air India 747 off the coast of Ireland that took the lives of 329 passengers and crew. A Sikh separatist group was suspected of the attack.

Many experts said they foresaw a growing likelihood of large-scale terrorist events. But they said that was based on the fear that well-organized and well-financed groups were increasingly close to obtaining weapons of mass destruction, including biological and nuclear weapons.

"Most people in counterterrorism were talking about the likelihood of a doomsday scenario involving germ warfare or nuclear weapons," said Juliette Kayyem, an expert on terrorism at Harvard's Kennedy School of Government who served on the National Commission on Terrorism.

"We feared that something could happen on a terrifying scale, but not that it would be done with conventional tactics — hijacking — that is reminiscent of the 1970's," she said.

Fighting terrorism became an important goal of the United States, Europe and Israel beginning in the early 1970's. Some Arab terrorists at the time tried to draw more international attention to their fight against Israel, while leftist groups, some backed by Soviet bloc nations, hoped to topple capitalism.

In 1972, Black September raided the Olympic village in Munich, Germany, killing 17 people including 11 Israeli athletes. Six years later, anticapitalism terrorism peaked in Europe with the kidnapping and murder of Aldo Moro, the former Italian prime minister.

Palestinian terrorists increased their attacks in the 1980's, as did violent Islamic fundamentalists. In that decade, the number of international terrorist incidents increased sharply, with a record 666 in 1987, according to the State Department.

The United States also became a prime target, although it was American people and facilities abroad, rather than at home, that drew fire. Two attacks in 1983, the bombing in April of the United States Embassy in Beirut and the suicide attack in October on the Marine barracks in that city, were aimed at driving the United States out of Lebanon.

In 1985, terrorists seeking to punish the United States for its support of Israel began killing American civilians. T.W.A. Flight 847 was hijacked in 1985, resulting in the death of a Navy diver on board. Palestinian terrorists killed Leon Klinghoffer, a 69-year-old American, after hijacking the Italian cruise ship Achille Lauro in the Mediterranean.

A dramatic change occurred in 1988, with the bombing of Pan Am flight 103. That is the first of what experts say became a pattern of revenge attacks designed to kill ever larger numbers of civilians. The attackers did not claim responsibility or issue communiqués stating demands.

Antigovernment domestic terrorists staged one such attack, the explosion in 1995 in Oklahoma City, the bloodiest on American soil until today. In Japan the same year, terrorists released poisonous gas in the Tokyo subway system, killing 12 commuters. The incident, which raised the specter of biological warfare by rogue groups, was attributed to the Japanese sect Aum Shinrikyo.

But the most ominous pattern of mass terrorist killings in the 1990's has been traced to the Saudi businessman Osama bin Laden and his allies.

These suspected incidents include the bombing of the World Trade Center in 1993; the bombing in 1995 of a Saudi National Guard training school in Riyadh that killed five American instructors; the truck bomb in 1996 that destroyed part of a housing complex used by American Air Force personnel in Dhahran, Saudi Arabia, in which 19 Americans were killed and 240 injured; the 1998 embassy bombings in Africa; and the attack on the Navy destroyer Cole last October that killed 17 American sailors.

"In the 90's we began to have indiscriminate, mass killings designed to shake the system to its core," said James A. Phillips, an expert on international terrorist at the Heritage Foundation. "They are ideologically or religious motivated, unlike the nationalist causes of the 80's, and most involve friends or associates of bin Laden."