History's greatest empires covered a fifth of the world, ruled hundreds of millions of people, and lasted anywhere from 100 years to over a millennium.

Empire, which comes from the Latin "imperare" meaning "to command," is a collection of states or ethnic groups united under one ruler or oligarchy.

It is a term that has been used to describe recent US foreign policy. Wrote filmmaker Oliver Stone and historian Peter Kuznick in a recent USA Today op-ed: "Obama is about to enter his second term as heir of George W. Bush's imperial strategy unless his latest foreign policy appointments signal significant change."

Each empire seemed unstoppable for an age, but in the end they were all consigned to history.