Class conflict
This would require a final showdown in the form of a social revolution.
One of the earliest analyses of the development of class as the development of conflicts between emergent classes is available in Peter Kropotkin's Mutual Aid. In this work, Kropotkin analyzes the disposal of goods after death in pre-class or hunter-gatherer societies, and how inheritance produces early class divisions and conflict.
Fascists have often opposed 'horizontal' class struggle in favour of vertical national struggle and instead have attempted to appeal to the working class while promising to preserve the existing social classes and have proposed an alternative concept known as class collaboration.

Twentieth and twenty-first centuries: 

Soviet Union and similar societies: A variety of thinkers, mostly Trotskyist and anarchist, argue that class conflict existed in Soviet-style societies. Their arguments describe as a class the bureaucratic stratum formed by the ruling political party (known as the nomenklatura in the Soviet Union), sometimes termed a "new class", that controls and guides the means of production. This ruling class is viewed to be in opposition to the remainder of society, generally considered the proletariat. This type of system is referred by them as state socialism, state capitalism, bureaucratic collectivism or new class societies. Marxism was already a powerful ideological power in Russia before the Soviet Union was created in 1917, since a Marxist group known as the Russian Social Democratic Labour Party existed. This party soon divided into two main factions; the Bolsheviks, who were led by Vladimir Lenin, and the Mensheviks, who were led by Julius Martov.
However, many Marxists argue that unlike in capitalism, the Soviet elites did not own the means of production, or generate surplus value for their personal wealth as the generated profit from the economy was equally distributed into Soviet society. Even some Trotskyists like Ernest Mandel criticized the concept of a new ruling class as an oxymoron, saying: "The hypothesis of the bureaucracy's being a new ruling class leads to the conclusion that, for the first time in history, we are confronted with a 'ruling class' which does not exist as a class before it actually rules."

United States: Class conflict was present in the earliest days of the U.S. with the struggles between slaveowners and slaves. Since the abolition of slavery, class conflict is more often noted in labor-management disputes. As far back as 1933, representative Edward Hamilton of the Airline Pilot's Association used the term "class warfare" to describe airline management's opposition at the National Labor Board hearings in October of that year. Apart from day-to-day forms of class conflict, there are also periods of crisis or revolution when class conflict takes on a violent nature and involves repression, assault, restriction of civil liberties, and assassinations or death squads.

The investor, billionaire, and philanthropist Warren Buffett, one of the wealthiest people in the world, voiced in 2005 and once more in 2006 his view that his class, the "rich class", is waging class warfare on the rest of society. In 2005 Buffet said to CNN: "It's class warfare, my class is winning, but they shouldn't be." In a November 2006 interview in The New York Times, Buffett stated that "[t]here’s class warfare all right, but it’s my class, the rich class, that’s making war, and we’re winning."
Noam Chomsky, American linguist, philosopher, and political activist, has spoken out on the topic of class conflict in the United States:

Well, there’s always a class war going on. The United States, to an unusual extent, is a business-run society, more so than others. The business classes are very class-conscious – they’re constantly fighting a bitter class war to improve their power and diminish opposition. Occasionally this is recognized... The enormous benefits given to the very wealthy, the privileges for the very wealthy here, are way beyond those of other comparable societies and are part of the ongoing class war. Take a look at CEO salaries....

Post-financial crisis: Class conflict intensified in the period after the 2007/8 financial crisis, which led to a global wave of anti-austerity protests, including the Greek and Spanish Indignados movements and later the Occupy movement, whose slogan was "We are the 99%", signalling a more expansive class antagonist against the financial elite than that of the classical Marxist proletariat.
In the speech "The Great American Class War" (2013), the journalist Bill Moyers asserted the existence of social-class conflict between democracy and plutocracy in the U.S. Chris Hedges wrote a column for Truthdig called "Let's Get This Class War Started", which was a play on Pink's song "Let's Get This Party Started." In a 2022 piece "America's New Class War", Hedges argues that increased class struggle and strikes by organized workers, often in defiance of union leadership, is the "one last hope for the United States."
Historian Steve Fraser, author of The Age of Acquiescence: The Life and Death of American Resistance to Organized Wealth and Power, asserted in 2014 that class conflict is an inevitability if current political and economic conditions continue, noting that "people are increasingly fed up [...] their voices are not being heard. And I think that can only go on for so long without there being more and more outbreaks of what used to be called class struggle, class warfare."

Arab Spring: Often seen as part of the same "movement of squares" as the Indignado and Occupy movements, the Arab Spring was a wave of social protests starting in 2011. Numerous factors have culminated in the Arab Spring, including rejection of dictatorship or absolute monarchy, human rights violations, government corruption (demonstrated by Wikileaks diplomatic cables), economic decline, unemployment, extreme poverty, and a number of demographic structural factors, such as a large percentage of educated but dissatisfied youth within the population. but class conflict is also a key factor. The catalysts for the revolts in all Northern African and Persian Gulf countries have been the concentration of wealth in the hands of autocrats in power for decades, insufficient transparency of its redistribution, corruption, and especially the refusal of the youth to accept the status quo.

Relationship to race: Some historical tendencies of Orthodox Marxism reject racism, sexism, etc. as struggles that essentially distract from class struggle, the real conflict. These divisions within the class prevent the purported antagonists from acting in their common class interest. However, many Marxist internationalists and anti-colonial revolutionaries believe that sex, race and class are bound up together. Within Marxist scholarship there is an ongoing debate about these topics.
According to Michel Foucault, in the 19th century, the essentialist notion of the "race" was incorporated by racists, biologists, and eugenicists, who gave it the modern sense of "biological race" which was then integrated into "state racism". On the other hand, Foucault claims that when Marxists developed their concept of "class struggle", they were partly inspired by the older, non-biological notions of the "race" and the "race struggle". Quoting a non-existent 1882 letter from Marx to Friedrich Engels during a lecture, Foucault erroneously claimed Marx wrote: "You know very well where we found our idea of class struggle; we found it in the work of the French historians who talked about the race struggle." For Foucault, the theme of social war provides the overriding principle that connects class and race struggle.
Moses Hess, an important theoretician and labor Zionist of the early socialist movement, in his "Epilogue" to "Rome and Jerusalem" argued that "the race struggle is primary, the class struggle secondary. [...] With the cessation of race antagonism, the class struggle will also come to a standstill. The equalization of all classes of society will necessarily follow the emancipation of all the races, for it will ultimately become a scientific question of social economics."
W. E. B. Du Bois theorized that the intersectional paradigms of race, class, and nation might explain certain aspects of black political economy. Patricia Hill Collins writes: "Du Bois saw race, class, and nation not primarily as personal identity categories but as social hierarchies that shaped African-American access to status, poverty, and power.": 44 
In modern times, emerging schools of thought in the U.S. and other countries hold the opposite to be true. They argue that the race struggle is less important, because the primary struggle is that of class since labor of all races face the same problems and injustices.

Chronology: Nationalist movements are not included.

See also: Class collaboration
Class consciousness
Classism
Critique of political economy
Criticism of capitalism
Critique of work
Economic stratification
Master-slave dialectic

References: 

Further reading: Abidor, Mitchell, ed. (2016). Death to Bourgeois Society: The Propagandists of the Deed. PM Press. ISBN 978-1629631127.
Adamic, Louis (1934). Dynamite: The Story of Class Violence in America (Revised ed.).
Antell, Gerson; Harris, Walter (2007). Economics For Everybody. Amsco School Publications. ISBN 978-1-56765-640-4.
Dahrendorf, Ralf (1959). Class & Class Conflict in Industrial Society. Stanford University Press. ISBN 0804705615.