Different sets of great, or significant, powers have existed throughout history. An early reference to great powers is from the 3rd century, when the Persian prophet Mani described Rome, China, Aksum, and Persia as the four greatest kingdoms of his time. During the Napoleonic wars in Europe American diplomat James Monroe observed that, "The respect which one power has for another is in exact proportion of the means which they respectively have of injuring each other.” The term "great power" first appears at the Congress of Vienna in 1815. The Congress established the Concert of Europe as an attempt to preserve peace after the years of Napoleonic Wars.