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...