a nice couple of sentences

Fuelled by grass, the Mongol empire could be described as solar-powered; it was an empire of the land. Later empires, such as the British, moved by ship and were wind-powered, empires of the sea. The American empire, if it is an empire, runs on oil and is an empire of the air. On the world's largest landmass, Iraq is a main crossroads; most aspirants to empire eventually pass through there.

From Ian Frazier (The New Yorker) via Kottke.