First, the US is not an empire. Empires take from their subject states, the United States gives out money, technology and protection.
There are different kinds of empires. Not all of them do their conquering as blatantly as Genghis Khan or the Spanish conquistas. The British Empire was a trade empire during it's first half, exporting technology, trading and bringing home wealth. Chinese empires have seldom attempted to expand or conquer.
Look at the Roman Empire or British Empire, they levied troops from their subject territories while ripping out the natural resources and taxing trade.
When the US entered Afghanistan, they bought war lords to help them combat the Taliban. The US doesn't tax trade but controls the rules of trade.
The US is an empire all right.
Different empires have different missions, but as imperial missions come, the American mission is pretty similar to the British and the Roman: To spread "civilization" in the name of a christian god. Look to the Spanish empire, the Chinese empires, Tsar Russia and the Soviet Union for other missions.
I really recommend reading Empires: The Logic of World Domination from Ancient Rome to the United States by Herfried Münkler, a great book which steers clear of the usual theories of imperialism and tries to go beyond, to explain the dynamics of empires, hegemonies and states.