Forces of nature have halted invading armies, prompted political change, and united bitter enemies.

The area where the tsunami caused the greatest loss of life, Indonesia's Aceh Province, had been under virtual martial law, largely closed to the outside world as 40,000 troops hunted separatists. Indonesia's military remains suspicious that Aceh rebels could exploit the chaos.

It remains to be seen how the Dec. 26 catastrophe will affect the political landscape, but tsunamis, volcanic eruptions, earthquakes, and other disasters have often deflected the course of history. Around 1600 B.C., the Santorini volcano sent a tsunami across the Mediterranean, devastating Crete, capital of the Minoan empire, its fleet, and its coastal cities. Fatally weakened, the empire was later conquered by the Mycenaeans of the Greek mainland, who established the model for Western culture.

Sudden and shocking as they are, earthquakes, volcanoes, and tsunamis are not the biggest forces in human history. Tiny microbes are more powerful. Plague undermined the medieval social order by killing a third of the European population in the 14th century, and native peoples of the New World fell to measles, smallpox, and other diseases borne by Spanish conquistadors and Puritans alike.

But winds and waves, even from average storms, can topple empires if timed perfectly--usually catching a navy at sea or an army on the march. As Bryn Barnard, the author of Dangerous Planet: Natural Disasters That Changed History, noted, typhoons in 1274 and 1281 (later dubbed the kamikaze or "divine wind") saved Japan by sinking Mongol fleets. The early onset of winter in 1812 crippled Napoleon's Grand Army and thwarted his invasion of Russia. Heavy rains, freezing mud, and below-zero temperatures caused German troops, still wearing their summer uniforms, to bog down just outside Moscow in December 1941.

KRAKATOA'S IMPACT

The 2004 tsunami will probably not end a civilization. But it did worsen the prospects for a nation's existence. The Maldives, dependent on fishing and tourism, lost habitable islands and a quarter of its 95 resorts, suffering damage equal to double its gross domestic product. A government spokesman admits that its future is in peril.

Several areas hit by the tsunami, particularly Aceh, contain some "very dangerous and unpredictable social cocktails," says Paul Saffo, director of the Institute for the Future, a San Francisco-area research group.

In 1883, more than 36,000 people were killed by tsunamis generated by the explosion of Krakatoa, a volcano on an Indonesian island between Java and Sumatra. Most of the island on which Krakatoa was situated was vaporized, and volcanic ash darkened the sky for days. According to Saffo, this cataclysmic event helped the cause of Muslim fundamentalists among the Indonesian nationalists already assassinating Dutch colonial planters and fighting their marines. That war was fought hardest in Aceh, which practices a militant Islam linked to the Arabian peninsula, rather than the gentler mix of animism, Hinduism, and Islam of Java, the island where Indonesia's capital, Jakarta, is situated.

POLITICS OF RELIEF

"There was a sense that the old gods had failed them," says Saffo, who expects the recent devastation to spawn new religious movements and cults in what is already an isolated rural area. He adds that the Indonesian government's response must be "swift, effective, and free of corruption or it will be a gift to the fundamentalists." Saffo argues that the American war on terror might be better served by outspending Islamic charities in Indonesia than by "pouring money into the sand" in Iraq.

Diane E. Davis, a professor of political sociology at the Massachusetts Institute of Technology, also foresees change in Aceh, though she speaks in terms more political than religious. The 1985 Mexico City earthquake, which she studied, hastened the end of 71 years of autocratic rule by the Institutional Revolutionary Party.

In relief efforts run by the Mexican military and police, aid packages were brazenly stolen, and police officers were assigned to rescue sewing machines from a collapsed garment factory while bodies lay in its rubble.

Indonesian soldiers now running relief in Aceh "could be the same ones that had just been murdering people," she says. "Imagine what that means on a face-to-face level."

By contrast, in war-exhausted Sri Lanka, citizens may see a contest. The government and the separatist Tamil Tigers each control a stretch of devastated coast: Who will do a better job helping the tsunami victims?

With the right amount of goodwill, disasters can be a unifying force, says Michael H. Glantz, an expert on early-warning systems at the National Center for Atmospheric Research. Perhaps the greatest success for a disaster, he says, was spawned by two earthquakes that took place just three weeks apart in 1999.

When one earthquake, near Izmit, Turkey, killed 17,000 people, the first country to send aid teams was Turkey's ancient enemy, Greece. When Greece in turn had a quake, Turkey reciprocated. Greece's foreign minister said later that the tragedies sent both nations a simple message: "We are all human." The warmer relations eventually led to talks over the disputed island of Cyprus and an end to Greek opposition to Turkey joining the European Union.