When are designations for historically significant phenomena treated with initial capital letters, and when are the names rendered with lowercase letters? Exceptions, as always, are available to confound us, but the rules are fairly straightforward.
Names of political and cultural periods or events are often capitalized in their original connotations, but when such nomenclature is used by extension in a generic sense, the designations are (usually) lowercased. For example, one should write, for example, “The arts and sciences flourished during the Renaissance,” but “The downtown district is experiencing a renaissance.” (However, to describe someone as well rounded in skills or talents, write “He’s a Renaissance man” even when he is not a contemporary of Michelangelo.)
The same distinction applies for such terms as “golden age” (“The Golden Age was the first of Hesiod’s Ages of Man,” but “Jazz music has experienced several golden ages”) and “belle époque” (“The period of peace and optimism in France in the nearly half century before World War I came to be known retrospectively as the Belle Époque,” but “They look back on that prosperous period as a belle époque”).
Similarly, one would write “China’s infamous Cultural Revolution was a decade-long time of great turmoil,” but “American society has undergone a cultural revolution of late,” and while references to the mid-twentieth-century tension between Western nations and the Communist Bloc capitalize “Cold War,” any such conflict without open hostilities is a cold war.
The Enlightenment was a specific cultural movement in Europe and Britain’s American colonies during the 1600s and 1700s, or a similar era in any one of several countries. Generic usage is as follows: “In the Western world, the concept of enlightenment in a religious context acquired a romantic meaning.” However, in specific usage, enlightenment is capitalized: “The Russian Enlightenment is a period in the eighteenth century in which the government in Russia began to actively encourage the proliferation of arts and sciences.”
Adjectives preceding names of political entities are often erroneously capitalized. No civilization has ever gone by the official name of Ancient Greece or Imperial Rome, for example; the first word in such designations is generally a mere descriptor and is therefore lowercased: “The course is a general overview of the history of ancient Greece”; “This essay will discuss the economic structure of imperial Rome.”