There's a medieval German proverb, "Stadtluft macht frei," which means "City air makes free." The city has always been the alchemical cauldron of human society, the great laboratory of human evolution, the birthplace and home of freedom itself. Now I've found a book, Welcome to the Urban Revolution by Jeb Brugmann, which explains exactly how and why the urban form's changing the human and even natural world. What's more, he even goes so far as to say that the world's cities are coalescing into a network strong enough that we can call it a single World City.
So what makes the urban form so inherently revolutionary? It's a simple matter of the mathematics of human association. In a hunter-gatherer tribe or agricultural village or small town, there's a limited number of people one can associate with. Society is tightly knit and highly conformist. The classic village or small town is notorious for its surveillance networks of town gossips. Social control is much easier in a society where most people are peasants and herdsmen. That's why an agricultural society almost always takes the political form of an absolute monarchy with a feudal hierarchy. This is the traditional form of civilization.
A large city is different. The modern city, which grew out of the medieval city republics modelled on ancient democratic Athens, was built by merchants out of trading posts. The larger and denser it is, the more potential associations a person can have. The more personal associations are possible, the greater diversity and dynamism the city generates. That's why in a capitalist and/or industrial society, the city is always the engine of the economy. Plus, the multiplication of association networks produces a levelling force that, even though capitalism has produced a neofeudal corporatist aristocracy of merchant princes and robber barons, makes the modern city an especially friendly environment for a democratic politics. The inevitable democracy of the city, as Brugmann calls it, makes it extremely difficult for tyrants to hold onto absolute power. For in urban politics as in capitalist economics, as Marx said, "all that is solid melts into air." Modern cities have always had a revolutionary effect on the societies they come to dominate, and all too frequently, whenever social evolution comes up against various forms of institutional stasis up to and including outright despotism, actual revolutions are the result and governments are overthrown. Brugmann gives as examples the fall of the Soviet Union and the Shah of Iran. The nominally Marxist overlords of the absolutist Soviet Empire found that out too late: once they had modernized and urbanized their countries, they found they had created a kind of democratic Frankenstein's monster that destroyed them.
And now the cities of the world, especially the Third World, are growing so large and dense so quickly that the "Great Migration" from the countryside to the cities is fundamentally transforming not only the nature of civilization itself, but even the natural environment planetwide. Though the world is currently organized into nation-states originating in the absolute monarchies that sprang up during the late Middle Ages, both economy and society are now organizing themselves by city regions and networks of cities. The city, not the obsolete nation-state and the international institutions (e.g., the United Nations, the World Bank, the World Trade Organization) built around it, is the fundamental unit of global civilization today. Sooner or later, the nation-state will have to go, and international institutions will have to become or be made interurban.
Which leads to the major consequence to the urbanization of the entire human race that not many people foresaw, and that's the consolidation of the cities of the world into a World City. The Cosmopolis has usually been dismissed as the pie-in-the-sky ideal of an ancient Greek philosopher named Isocrates. But the industrial and information revolutions have produced a corresponding urban revolution so huge and all-transforming that Isocrates' vision of the World City is finally becoming our reality.
But this has its own problems. For one thing, world industrialism is producing such a mountain of waste that it threatens to destroy the global ecology. Another problem is economic and has to do with urban development: with the recent global economic boom, many countries have simply imported a commoditized urban form, the American suburban "edge city", that is merely plopped right down in place without any care for the surrounding urban context. In many countries, this process (misnamed "urban renewal" in the US) requires massive "slum clearance" and relocations, which tends to cause huge riots. This happens all the time in China, for example.
Marshall McLuhan famously spoke of the "global village." He was only half right. For that matter, the neoliberal globalists are also half wrong. They thought that global capitalism was sufficient to unite the world. The spoke of the world market. What is actually emerging as the true face of globalization is the unification of all the world's civilizations into a single World City, in which every city is a neighborhood and every city region or network is a district. The world is becoming a single city, with all the consequences that entails. This really is a new stage of history. It is, well, revolutionary.