I agree with the many postings considering the years after WWII and the Cold War as the peak of US influence. However, we should not forget that 1945-1965 was not a Golden Age for all people within the US, and we should not forget the significant progress made in the decade after this period.
In contrast to many others, I am optimistic in regard to America's future. It will continue to draw the most talented immigrants. I don't see why its population and its wealth shouldn't grow in the decades to come, if the right decisions are made concerning education, the economy, military engagements, the distribution of wealth, and civil rights. But these questions have to be answered in every country by each new generation, so it is nothing special for the United States.
America's relation to China and India will be of great importance. I hope that these powers will perceive each other as peaceful competitors and partners, and not as enemies.