At the moment the US posesses unrivalled hegemony; they're top dog, and many Americans (including their President) seem to revel in rubbing it in. The problem is that it isn't going to last. A quick analysis by MaxSpeak shows that the "unilateral moment" is likely to pass sooner rather than later as the Chinese economy grows. Matthew Yglesias looks at this and concludes that
The policy goal of indefinite American military hegemony is simply incompatible with the goal of continued growth in Chinese and Indian prosperity. A policy of trying to deliberately perpetuate the impoverishment of 3 billion human beings would be morally problematic, as well as pragmatically hard to pull off. Thus, no indefinite American military hegemony.
Right-wingers like to pooh-pooh the Chinese economy, claiming that growth rates are overstated, that it's all just flash in the pan, and that China will inevitably collapse - meaning no threat to American dominance (at the same time they're usually frothing at the mouth about the "ChiCom" danger - which is a great example of doublethink in action). But even if they're right, and if China descends into civil war again (as it did for half the twentieth century), the underlying point remains: nothing lasts forever. America's empire, like all empires, will eventually fall. America would be wise to plan for this eventuality. Yglesias again:
we really should be spending the next five (and ten, and fifteen) years on trying to make sure that the global system is on a trajectory such that we can continue to be comfortable with that trajectory once we are no longer hegemonic.
Instead though, the US is trying to undermine the present international system in favour of a "winner take all" model. But if they succeed in this, they are going to be very uncomfortable indeed when someone else is eventually the winner...