We don't really have any enemies...
Sure, there is Iran, but that's an enemy of our own making. And they are extremely weak - we could take them out quite easily if they ever truly threatened us.
Of course there is Al Qaeda and Islamic terrorists, but those too are enemies of our own making. If we minded our own business, the Islamic terrorists would be too busy slaughtering each other to ever attack us.
China is on the rise, but it'll take a long time and many economic reforms for them to ever pose a serious threat to us.
India is rising too, but they are quite friendly to American interests. And again, they need serious economic reform if they're ever going to truly rise to our superpower status.
Russia will never be more than a regional power in Eastern Europe - they lack the long-term potential of China and India.
The USA is dominant and could remain so for the long term if it further liberalized its economy and labor market by cutting taxes, cutting spending, deregulating, and allowing more people into the United States. The combination of strong economic growth and population growth would make the United States untouchable for years to come. Of course, avoiding creating more enemies (as we have done with Iran) would be necessary in order to keep amassing power. Unfortunately, it seems like the US's greatest enemy is itself - if the US continues its path toward bankruptcy, statism, and inflationism, it will ultimately destroy itself and its world power.