Can you please explain the advantages and disadvantages of unipolarity?

More
specifically, one should regard the advantages and disadvantages of unipolarity in regard to the
exercise of military power. 

The United States, for example, outspends every
other industrialized nation combined on the maintenance of its defense. This gives the United
States an advantage that it has enjoyed since the Second World War: the ability to police
conflicts in different parts of the world, particularly nations "threatened" by
Communism or a totalitarian power. The US's gargantuan military power has given it the ability
to ensure, or pursue, democracy throughout the worldeven in nations that...