(FORTUNE Magazine) – It's terrific that the 20th century is ending as it began, with democratic capitalism ascendant. Prospects for a less bloody, more prosperous world have rarely been brighter. But it's also worth recalling that the past 75 years were by far the most destructive in human history. Mankind's darker impulses could still foul the future in one or more of the following ways. -- SUPER TERRORISM. Advances in military technology continue to put ever greater firepower into the hands of Colombian drug dealers, Middle Eastern fanatics, and antisocial groups of all stripes. ''What will we do when the terrorists get nuclear or chemical or biological weapons?'' asks Louis Finch ( of the Rand Corp. ''Inevitably, they will.'' The ability of organized states to combat these private armies shows no such growth. A lot more international cooperation -- and backbone -- will be needed to turn this dark tide. -- THE LONG GOODBYE: While Marxism beats a swift retreat from Eastern Europe, it's hard to see signs of an about-face in Peru, Cambodia, Afghanistan, Angola, El Salvador, or innumerable other Third World hot spots. And how soon will China, with a quarter of the world's population, feel freedom's breeze blowing? Says Constantine Menges, a resident scholar at the American Enterprise Institute in Washington: ''We may see a North-South split. In the Northern Hemisphere, we'll be saying, 'What cold war?' In the South they'll be saying, 'What liberalization?' '' -- BACK TO THE FUTURE. With the thawing of superpower relations and the breakup of the Soviet empire, dozens of ancient regional conflicts and long- buried national aspirations could resurface. Says Rand Corp. analyst Jeremy Azrael: ''We could return to the 19th century. The history of Balkan nationalism was never finished. Yugoslavia is close to civil war. Hungarians are furious over Rumania's treatment of ethnic Hungarians there.'' Back in the U.S.S.R., dissident Azerbaijanis have been using hand-fired rockets against trains crossing from the neighboring republic of Armenia. None of these conflicts would likely drag Western powers into a major war. But violence could soar east of the Elbe. -- A NEW IDEOLOGY ARISES. Capitalist democracies have flourished by proving better than any other system in giving people what they want. Right now, what most people want is a constitution and a VCR. That could change. As the revival of Islamic fundamentalism in the Middle East since the late 1970s powerfully demonstrates, the pull of nonrational, uneconomic impulses remains strong. True, Islam holds limited appeal in the West. But something else might prove compelling. Something like the Green movement. Before you say it can't happen here, ponder the public outcry that followed the recent oil spill in Valdez, Alaska. Observe the steely glint in the eyes of Greenpeace activists. Now imagine some big catastrophe -- a sharp rise, say, in ozone-related cancers -- that gives popular fears of an environmental apocalypse a giant push. It's just possible that the toughest challenger to democratic capitalism has yet to surface.