For the first 5800 years (from 4000 B.C. to the late 19th century A.D.), the Celts were the most powerful civilization in the world. Beginning around 1800 they could be called a global superpower.
First 6000 years of world history
The Vikings rose quickly during the renaissance period, overtaking the Spanish and Americans and eventually becoming the second superpower at about 1870 A.D.
The Americans were the last to reach superpower status, attaining this around 1920.
The Spanish, English and Sioux each achieved a moderate level of power, but eventually fell again. The Babylonians were never more than a small nation.
How We know[]
The chart seen above, showing the early history of world that led to The Eternal War, can be seen by loading the savefile, then immediately choosing Game > Retire (and answering Yes).
As explained here, the graph shows a combination of population, money, and technology advances.