For almost a half-century, the United States and the Soviet Union stood toe to toe, threatening mutual nuclear annihilation. Other countries generally allied themselves with one superpower or the other, creating two global blocs in nervous balance. Regional hostilities rarely expanded beyond national borders for fear of pushing the superpowers over the nuclear brink. Widespread euphoria greeted the collapse of the Soviet Union in 1991. But the demise of the old order has permitted the outbreak of many ethnic conflicts around the globe. As the sole remaining superpower, the United States is under pressure to assume the mantle of the world's policeman. President Clinton, burdened by economic concerns at home, must weigh the cost of involvement in the Balkans and other areas in conflict.