Personally, I think it has been the successful infiltration of the Democratic Party by Marxists, radicals and determined socialists. This didn't happen over night. The greatest nation on earth has been poisoned slowly over time, though a patient gradualism, in which our traditions, institutions and way of life have been slowly displaced. The frog has been slowly boiled, with the heat raised only a degree at a time, unnoticed until it is too late. Frog soup, anyone?
The Italian Communist Antonio Gramsci advocated this gradualist approach to undermining the west, while he was sitting in an Italian prison in the 1930's. He advocated the destruction of Christianity so it could be replaced by a nearly religious faith in the power of the state. He wished for a "long march through the institutions," a take-over by the left of the media and academia. The leftist press would publish only such news as would move the revolution forward, and the university would thoroughly immerse students in collectivist propaganda.
Gramsci advocated "cultural hegemony," which is most often referred to as "cultural Marxism" today. Per Wikepedia:
Cultural hegemony is the philosophic and sociological theory, by the Marxist philosopher Antonio Gramsci, that a culturally diverse society can be dominated (ruled) by one social class, by manipulating the societal culture (beliefs, explanations, perceptions, values) so that its ruling-class worldview is imposed as the societal norm, which then is perceived as a universally valid ideology and status quo beneficial to all of society, whilst benefiting only the ruling class.Having the "right" beliefs is rewarded in such a society, through social acceptance, better grades in school, and more opportunities for employment. Having the "wrong" beliefs can result in social ostracism and loss of employment. In the worst case, one might have his name publicly blacklisted, where would-be employers can reject him and media can denounce him to the public at large (the Southern Poverty Law Center maintains such a list). Such ostracism effectively stops private inquiry, research and publication that might present facts contrary to the leftist worldview.
Cultural Marxism works to slowly desensitize the average citizen from his bourgeois beliefs in the church, in the institution of marriage, merit-based employment and admission to the university. The church is undermined through ridicule, as in the immersion of a crucifix in a jar of urine as a form of high art. Marriage is undermined through the advocacy of gay marriage now and polygamy later. A sense of guilt is inculcated in students for being white, capitalist, prosperous, American and free, as these conditions are effectively (though falsely) connected to poverty and misery in the third world. Since we are [allegedly] responsible for this poverty and misery, we are obligated to hand over our wallets to leftist politicians for delivery to the third world victims.
We are further encouraged to go along through propaganda disguised as science: man-made global warming will destroy the planet unless we turn over the means of production to much greater government control. So we introduce yet another disincentive to resistance: fear. Nature will kill you if you do not surrender individual freedom to a new super-government.
Beliefs and behavior are also regulated through informal speech codes imposed in both academia and the media: one does not refer to the race of rioters or the ideology of terrorists when writing news reports. One does not hold anyone accountable for violence or anti-social behavior, and to do so constitutes a terrible condition known as racism...for which one will be blacklisted, ostracized and unemployed.
All of this leftwing activity points to one grand scheme: the massive redistribution of wealth from those who produce it to those who do not. We are pressured to go along through guilt and fear. Guilt of our own success, fear of social ostracism, fear of "climate change," fear of violence by those we are entreated to appease.
Now a writer named Frank Miele explains the long term effects of cultural Marxism in his article "Is American decline the product of chance or of design?" Before the counter revolution can begin, however, we need to understand how we got to where we are.