Complex sentence: Since 1991 the United States has been the world's dominant military, economic, social, and political power, the United Nations was not designed for such a unipolar world with a single superpower, and conflict between an ascendant U.S. and other UN members has increased.
Split into simpler sentences: Since 1991 the United States has been the world's dominant military, economic, social, and political power. The United Nations was not designed for such a unipolar world with a single superpower, and conflict between an ascendant U.S. and other UN members has increased.