American government leading the world

Can and should America still lead the world? Many Americans are clamoring for the United States to pull back from trying to influence international affairs in favor of greater attention to domestic issues. What challenges does the United States face in trying to lead the world in this 21st century? What advice would a political realist have for the architects of U.S. foreign policy? What advice would a political liberal have for the architects of U.S. foreign policy? Can the United States lead the world in the 21st century the way it did in the latter 20th century?