Simply because the US is the most powerful nation in the international arena today does not automatically imply that it is the most capable of bearing the responsibility of leadership and that it should consequently be called upon to bear that mantle.
Leadership calls for additional qualifications.
First, the country should be willing to exercise leadership and capable of sustaining the responsibilities and consequences of its role. Second, it should be morally and ethically qualified. Finally, it should enjoy the confidence and respect of the international community, or at least of the majority of the world's peoples, so that its authority emanates from reverence for its status and support for its principles rather than from the exercise of brute force and vengefulness. I do not believe that the US meets these conditions at present. A cursory glance at US foreign policy positions and practices since the fall of the Soviet Union and the end of the Cold War is sufficient to realise how inconsistent and haphazard US policy is.