Postmodernism is a political ideology that developed in the late 20th century. It is characterized by subjectivism and relativism—a deconstruction of the Age of Enlightenment ideas of reason and objectivity—with the root idea being "everything is a social construct." It can be thought of more as a strategy for asserting political and economic power, than a true belief system.
The term counter-enlightenment might be better than the cryptic term postmodernism, which leaves everyone scratching their head. A circular problem present in discussing postmodernism is that most people don't know what it means, because it doesn't mean much of anything! When we just call it what it is—an anti-reason cult (simplified); the average person can understand. Postmodernism also has a lot of overlap with the humorous theory of Idiocracy (the 2006 film), and the frightening phenomenon of ideocracy (a type of power-driven ideology that thru totalitarianism and/or populism dominates society thinking, and seizes control of government).
THE CLAIMED BELIEFS ARE A PLOY, IT IS ABOUT POWER
It is important to understand that postmodernists don't truly believe everything is subjective, and thus nothing is real. We know this because after "disproving" existing reality, they choose a very specific new reality. And so they just say they believe in subjectivism as a strategy to destabilize established thinking, ignore scientific evidence (especially biology which falsifies their central theory), and tear down the pillars of Western establishment—so that they may recreate society with their own vision.
But what is that vision? A socially-engineered "utopia" of anti-meritocracy, tyranny of the minority, doublethink, and control over speech/behavior. This has led critics to point out that postmodernism is really just an evolved form of Marxism—since the old form was disproven by reason and results, they simply moved their ideology post-reason, post-reality, and even post-dialog in order to end the discussion forever. However I would also point out it has similarities to Nazism—the creation of a new "Aryan race" who have special status/rights due to their genetics (women, non-whites, etc). Additional power is gained by these "identity groups" by claiming they are "not equal yet," into perpetuity. Unlike Marxism which, in theory, is about the masses rising up and owning the means of production, postmodernism is about creating a new aristocracy (rule by the few). In the end postmodernism is about raw power.
Once we understand it is all just about power—with no interest in reason, facts and laws—then we can see it more clearly as a totalitarian movement of the left. Totalitarian regimes often involve a cult, and pursue the subjugation of the majority of the population (in this case by targeting majority "identity groups") on behalf of a numerically insignificant elite (minority groups) who tell everyone else how they should think, and what they can do/say. It is a bizarre "tyranny of the minority" situation which should be difficult to maintain in a republic such as the United States (debatable, perhaps pseudo-republic), yet which has nonetheless taken hold.
HOW COULD THIS HAVE SPREAD SO FAR, AND FAST?
Although the ideology was authored decades earlier (see Jacques Derrida and others), it didn't really start spreading until the early 90's, when it hit university campuses. By the mid/late 90's warnings were sounded by people active in those educational environments (such as Stephen Hicks, Peter Thiel, and Theodore Kaczynski in his infamous 1995 essay). In the following decades it spread outward from universities to corporations and politics (confusingly also top-down from corporations and politics, this is coming from multiple directions, sweeping across the whole society). At this point it almost seems the establishment ideology, across the West.
A key to grasping how this ideological cult has taken over, resides in understanding corporatism (discussed here). In America, the large corporations have undeniably seen value in postmodernism. We know this because they have used their corporate media arm to widely spread the ideology, and have strictly enforced it thru their individual HR departments. My theory on why—as the masses inevitably begin to struggle and revolt against corporatism due to the externalities of predatory profit maximization, corporations (which have close ties to government) can opportunistically use postmodernism as an efficient way to suppress them. Again, this is accomplished thru the suppression of majority groups, using the facade of "freeing minorities" from oppressive majority group tyranny (an imaginary enemy fabricated using their media assets). And as the propaganda succeeds, the citizens themselves police and report on each other, making it cost effective. Plus the ideology has the added utility of "sounding good," it humanizes the corporation by demonstrating how it "cares so much about the oppressed," while it carries out said oppression.
As far as its spread into politics—while postmodernism began as a left-wing ideology, it has now infected both the left and right. Obama was perhaps the first postmodernist president, who acted as the seemingly reasonable front man, counter-balancing the anti-reason movement. Hillary ran on the essentially the same postmodernist package of ideas, but was too transparent to be effective. As with corporations, politicians use postmodernism because it projects the image that they "care so very much," and thus opponents must be heartless. Left-wing media (almost all mass media) supported and spread these ideas, while attacking dissenters. There was a huge bandwagon effect (a trait of ideocracies). But there was a weakness in this strategy—having altered the environment so that objectivity and facts no longer mattered, they left this environment open for use by anyone, not just the postmodernist-left. Trump entered politics using this existing battlefield to his own advantage—if facts don't matter, and it's just about subjectivity, then my subjectivity is just as good as yours! And so the postmodernist-right was born, new right-wing media emerged, and the age of warring postmodernist cults began (bye bye enlightenment age!). Note it was only at this point that the media started calling it "post truth" and "fake news," but only because the other side was doing it.
It is thru this coalition of the ideological postmodernists, pragmatic corporatists, and opportunistic politicians that this counter-englightenment movement has been able to broadly seize power.
Perhaps every society that has existed has been ruled by a small percent, and "change" to the system only ends up being a new ruling small percent. As the saying goes "meet the new boss, same as the old boss." But with capitalism as least the oligarchs were selected largely thru meritocracy, and thus possessed a level of competence. The ultimate problem with postmodernism is its anti-meritocracy nature—and so the new oligarchs can't possibly maintain the structures over which they seize power.
Most conservatives, classical liberals, and free-market thinkers would agree with this view—that attempts to remove the natural element of meritocracy from human society (a fixture in the animal kingdom related to competition, survival of the fittest, and natural selection, which has a biological anchor and thus can not be removed) eventually fails by its own success.
But there is an additional layer everyone is missing. While I would say the above capitalist thinking is true, it is based on the assumption that America is still a capitalist and/or free-market system, and that the oligarch's ownership is still equivalent to control. But I see no basis for that assumption under America's current economic system—corporatism. Corporatism is a rules-based system which removes human judgement, and so even the oligarchs end up locked into moves dictated by the massive inflexible system.
And so the postmodernists are struggling for control of the corporate-government apparatus, through which they believe they will in turn gain control over society. But I would say control flows the other way—they are just the pawns of corporatism, which uses their ideology, as a strategy to control the masses and government.
POSTMODERNISM ANALYZED BY CRITICS:
In this video Jordan Peterson discusses the post-logic and post-discussion political cult of postmoderism, which is spreading across the United States, Canada, and Europe...
I'd also recommend this detailed history and analysis of postmodernism by Stephen Hicks, who was one of the first to identify the movement in the late 90s...
Postmodernism. Encyclopedia Britannica.
Hicks, S. (2004). Explaining Postmodernism: Skepticism and Socialism from Rousseau to Foucault. Ockham's Razor.
Kaczynski, T. (1995). Industrial Society and Its Future. The New York Times.