With the fall of the Roman empire in 476, the Church became the greatest power in the Western world. Its hierarchy, & the system of local churches or dioceses, were the only cohesive factor in the former empire. Roman coordination of the Western world disappeared, & with it efficient systems of roads, transportation, communication, aqueducts, fresh running water to the cities, etc. The Silk Road through Asia to China was disrupted. The West became isolated. The West itself broke down politically, from an empire uniting nations, to feudal duchys ruled by lords who were largely independent of their Kings.
The Church had a philosophical interpretation of this chaos. Medieval society was composed of four classes: priests; knights the nobles, the aristocrats, the military class; the middle class -- the merchants; & the workers -- the serfs -- in that order- This institutionalization of a decadent social structure lasted for almost a thousand years, until the beginning of the Renaissance around 1400.
The ideal of a Christian empire developed amid the chaos. The church was a spiritual empire, headed by the pope. But ideally, there should also be an earthly empire, ruled by a Christian emperor subject to the pope. This had in fact been the case in the late Roman empire. From Constantine's conversion to Christianity in 312, all emperors but one were Christians under the influence of the Church. The exception, Julian the "Apostate" was a pagan who became emperor in 351, & tried to bring the empire back to paganism, but he only lasted two years before falling in battle. Under the influence of Aristotle, the ideal of a Christian world emperor developed. Julius Caesar was looked back at as being the founder of the earthly empire, just as Jesus was the founder of the spiritual empire, & the lineage of the popes. Thus in Dante, the lowest circle of Hell is reserved for Satan, who rebelled against Jehovah; Judas, who betrayed Jesus; & Brutus & Cassius, who "betrayed" Caesar (actually they assassinated a tyrant who had destroyed the Roman republic, in an attempt to restore it, which failed). But Dante was an active propagandist for the ideal of the universal Monarch.
A real attempt was made to create such an empire. In 800, pope Leo III crowned Charlemagne as the first Holy Roman Emperor. The Holy Roman Empire lasted until 1806, but it never equaled the real Roman Empire, or fulfilled the dreams of Dante & others.
Throughout the Middle Ages, there was a struggle for power between the Church & the aristocrats, including the Holy Roman Emperor - Some of the stronger emperors succeeded in gaining independence from the pope, beginning with Frederick Barbarossa in the late 12th century, anticipating the modern separation of church & state.
In the late Middle Ages, the middle class, the merchants & bankers, began to rise to vast wealth &, consequently, power. he priests & aristocrats united against the merchants. In the Renaissance, feudalism faded as the modern nation states developed.
The democracy of the ancient city-states was revived in the late Middle Ages in cities such as Florence. Modern democracy began to evolve in England. In the Age of Enlightenment, philosophers such as Locke & Rousseau praised the benefits of democracy -- Rousseau pointing to his native Geneva, where he couldn't bear to live. Paine & Jefferson were inspired by Locke & Rousseau, in developing the philosophical foundation for US democracy. The peasants of France were inspired by Rousseau in over-throwing Louis XVI, & they were soon being led by Rousseauan intellectuals such as Robespierre. But Robespierre was killed, Napoleon took power, & the contemporary bourgeois age began.