With last Tuesday's horrific assault,"terrorism" has become the new American watchword and scourge. Yet as George Bush declares a war on terrorism," neither he nor anyone else has defined what terrorism is or where it comes from. Doing so may help us to face a frightening reality: the catastrophe of September 11 represents terrorism of a new, more challenging kind.

Not all political violence, it should be specified, amounts to terrorism. Committed by stateless organizations against established powers, terrorism doesn't attack military centers to seize power, like guerrilla warfare. It specifically targets random, unsuspecting victims to publicize a grievance and sow panic among the strong.

The word comes from the French Revolution's"Reign of Terror" (1793-94), when Robespierre's Jacobins executed 12,000 people deemed enemies of the Revolution, often for flimsy reasons. Since the Jacobins ran the state, we wouldn't call them terrorists today. Still, their vision of a violent purge in the name of Utopia provided a model for later insurgents.

Over the next century the Jacobin spirit infected Russia, Europe and the United States. Radical anarchists - Leon Czolgosz, who killed William McKinley in 1901; Alexander Berkman, who shot steel magnate Henry Frick in 1892; the Russians who assassinated Tsar Alexander II in 1881 - targeted powerful leaders to foment popular revolution. Alongside bombings such as the one at Chicago's Haymarket in 1886, these killings created publicity and popular panic. Yet the anarchist revolution never came.

In the mid-20th century, native peoples from Egypt to Vietnam rebelled against colonial regimes. They too used dramatic acts of destruction - called terrorism by some - to win attention. But it was Algeria's Front de Libération Nationale, seeking liberation from France, that defined modern terrorism, deliberately spilling the blood of random French civilians.

After France executed two Algerian rebels in 1956, the FLN slaughtered 49 Frenchmen in three days. FLN terrorists bombed beachside cafés where they knew families would perish. They wanted to raise the price of colonialism to intolerable levels. They did.

Their success inspired others: Basque and Quebecois separatists, Palestinian and Irish nationalists, Marxist cabals in Africa and Latin America. By the 1960s, the killing of civilians to sow fear and secure political gains was rampant, even in developed nations - from the Weather Underground in the U.S. to the Marxist Baader-Meinhoff Gang in West Germany to the Red Brigades in Italy.

But Western terrorism of the '60s and '70s (like recent right-wing variant of Timothy McVeigh) paled next to the violence in the Middle East. It need not diminish Yasser Arafat's recent peacemaking moves to recall that for years his Palestinian Liberation Organization unabashedly murdered civilians amid some of the world's most shocking deeds.

The constituent groups of Arafat's PLO pioneered hijacking and hostage-taking to win global recognition for their statehood demands. The 1972 murder of 11 Israeli athletes at the 1972 Munich Olympics and the 1985 killing of the wheelchair-bound American Leon Klinghoffer during the commandeering of the Achille Lauro remained etched in public memory. Nonetheless, they helped highlight Palestinian grievances.

Like such contemporaneous groups as South Africa's African National Congress, the PLO's goals were political, not religious. Courting world opinion, they realized that if you live by the car bomb, you die by the car bomb; terrorism could alienate the very people whose respect its perpetrators sought. Indeed, Arafat, Nelson Mandela and others had to distance themselves from terror to prove they could lead new governments. As the PLO's terrorism abated, Islamic fundamentalism swept the Middle East. Starting in 1979--the year of the Israel-Egypt peace agreement, the Iranian revolution, and the Soviet invasion of Afghanistan--Mideast terrorists began hailing from overtly religious groups: Hamas, Hezbollah, Islamic Jihad, Algeria's Islamic Armed Group.

Espousing a warped vision most Muslims emphatically reject, many believed the United States to be the symbol and stronghold of satanic Western values. Inevitably, some, such as the Saudi exile Osama bin Laden, made the U.S. their actual target.

Unlike the violence of the 1960s and '70s, the attacks on the U.S. are not secular or Marxist. Unlike the nationalist terror of the IRA or the FLN, they aren't aimed to achieve a negotiated political settlement. They are neither part of a war of rebellion, nor a form of left-wing anarchism, nor a barbarous exercise of state power.

This new terrorism springs from an unswerving conviction that to destroy America is to do God's work. Since it doesn't play to world opinion, world opinion cannot act as a brake upon it. And since it failed to destroy America, we should expect it will strike again.

A version of this piece appeared recently in Slate.com.