A major challenge that the US faces right now is globalization. Globalization is a major challenge to the US because it threatens our global economic dominance and causes serious upheaval in our domestic economy and politics.
Globalization is hurting the US economy to some degree. It is reducing the growth in American jobs and it is allowing other countries to catch up to us to a great extent. This is a challenge to us because it forces us to do at least two important things. First, we must figure out how to compete on a global scale. This may, for example, mean revamping our educational system and/or our tax system. Second, we must figure out how to deal politically with the fact that our country is experiencing growing inequality due in large part to this globalization. Much of the anger that is being expressed in politics today is a result of this fact.
Thus, globalization is a major challenge because it forces us to think about how we might change our society and our economy to remain competitive while still managing to maintain a modicum of equality among all Americans.