While the US is undoubtedly still the most powerful country both economically and particularly militarily in the world, it will not be long for that situation to change.
Economically, there are already some people who argue that China has already overtaken the US as the world largest economy.
Militarily, the power is largely though not exclusively dependent on economic power. Arguably, there may be some lags between the rise of a country’s economic power and it’s military power, though the lag may not necessarily follow only one direction.
With the increasingly more rapid speed in catching up economically, how long a country can sustain the number one status does not depend on the existing one alone but the new comers.