Class conflict
It is by this superior knowledge of their own interest that they have frequently imposed upon his generosity, and persuaded him to give up both his own interest and that of the public, from a very simple but honest conviction, that their interest, and not his, was the interest of the public.In the final words of the first book, he suggested how the public should consider the political agenda of this class:The proposal of any new law or regulation of commerce which comes from this order, ought always to be listened to with great precaution, and ought never to be adopted till after having been long and carefully examined, not only with the most scrupulous, but with the most suspicious attention. It comes from an order of men, whose interest is never exactly the same with that of the public, who have generally an interest to deceive and even to oppress the public, and who accordingly have, upon many occasions, both deceived and oppressed it.

Karl Marx: Karl Marx (1818–1883) was a German born philosopher who lived the majority of his adult life in London, England. In The Communist Manifesto, Karl Marx argued that a class is formed due to changes in the mode of production. Members of a class may become aware of their position within it, achieving what is known as class consciousness. For the working classes this happens when their members become aware of their exploitation at the hands of the ruling class. According to Marx, the working class then takes action against the ruling class, and vice versa.
What Marx points out is that members of each of the two main classes have interests in common. These class or collective interests are in conflict with those of the other class as a whole. This in turn leads to conflict between individual members of different classes.
Marxist analysis of society identifies two main social groups:

Labour (the proletariat or workers) includes anyone who earns their livelihood by selling their labor power and being paid a wage or salary for their labor time. They have little choice but to work for capital, since they typically have no independent way to survive.
Capital (the bourgeoisie or capitalists) includes anyone who gets their income not from labor as much as from the surplus value they appropriate from the workers who create wealth. The income of the capitalists, therefore, is based on their exploitation of the workers (proletariat).
Not all class struggle is radical, violent or aggressive, as in the case of strikes, lockouts and workplace sabotage. Class antagonism can also surface in minor acts of pilferage, low worker morale, and individual worker resistance to petty authority and hoarding of information. On the employers' side, lobbying for anti-union laws and against minimum wage increases, and hiring union-busting legal firms are expressions of class antagonism. On a larger scale, workers engage in class struggle when they agitate for systemic change via socialist or populist parties.
But not every instance of class struggle from below constitutes a grave threat to capitalism, or even to the authority of an individual capitalist. A narrow struggle for higher wages by a small sector of the working class, what is often called "economism", hardly threatens the status quo. In fact, by applying the craft-union tactics of excluding other workers from skilled trades, an economistic struggle may even weaken the working class as a whole by dividing it. Class struggle becomes more important in the historical process as it becomes more general, as industries are organized rather than crafts, as workers' class consciousness rises, and as they self-organize away from political parties. Marx referred to this as the progress of the proletariat from being a class "in itself", a position in the social structure, to being one "for itself", an active and conscious force that could change the world.
Marx focuses on the capital industrialist society as the source of social stratification, which ultimately results in class conflict. He states that capitalism creates a division between classes which can most clearly be seen in factories. The proletariat is separated from the bourgeoisie because production becomes a social enterprise. Contributing to their separation is the technology that is in factories. Technological advances de-skill and alienate workers as they are no longer viewed as having a specialized skill. Another effect of technology is a homogenous workforce that can be easily replaceable. Marx believed that this class conflict would result in the overthrow of the bourgeoisie and that the private property would be communally owned. The mode of production would remain, but communal ownership would eliminate class conflict.
Even after a revolution, the two classes would struggle, but eventually the struggle would recede and the classes dissolve. As class boundaries broke down, the state apparatus would wither away. According to Marx, the main task of any state apparatus is to uphold the power of the ruling class; but without any classes there would be no need for a state. That would lead to the classless, stateless communist society.

John Stuart Mill: Adam Smith was not the only classical liberal political economist concerned with class conflict. In his Considerations on Representative Government, John Stuart Mill observed the complete marginalisation of workmen's voices in Parliament, rhetorically asking whether its members ever empathise with the position of workmen, instead of siding entirely with their masters, on issues such as the right to go on strike. Later in the book, he argues that an important function of truly representative government is to provide a relatively equal balance of power between workmen and masters, in order to prevent threats to the good of the whole of society.

During Mill's discussion of the merits of progressive taxation in his essay Utilitarianism, he notes as an aside the power of the rich as independent of state support:People feel obliged to argue that the State does more for the rich than for the poor, as a justification for its taking more [in taxation] from them: though this is in reality not true, for the rich would be far better able to protect themselves, in the absence of law or government, than the poor, and indeed would probably be successful in converting the poor into their slaves.

Max Weber: Max Weber (1864–1920) agreed with the fundamental ideas of Karl Marx about the economy causing class conflict, but claimed that class conflict can also stem from prestige and power. Weber argued that classes come from the different property locations. Different locations can largely affect one's class by their education and the people they associate with. He also stated that prestige results in different status groupings. This prestige is based upon the social status of one's parents. Prestige is an attributed value and many times cannot be changed. Weber stated that power differences led to the formation of political parties. Weber disagreed with Marx about the formation of classes. While Marx believed that groups are similar due to their economic status, Weber argued that classes are largely formed by social status. Weber did not believe that communities are formed by economic standing, but by similar social prestige. Weber did recognize that there is a relationship between social status, social prestige and classes.

Charles Comte and Charles Dunoyer: Charles Comte and Charles Dunoyer argued that class struggle came from factions that managed to gain control of the state power. The ruling class are the groups that seize the power of the state to carry out their political agenda, the ruled are then taxed and regulated by the state for the benefit of the ruling classes. Through taxation, state power, subsidies, tax codes, laws, and privileges the state creates class conflict by giving preferential treatment to some at the expense of others by force. In the free market, by contrast, exchanges are not carried out by force but by the non-aggression principle of cooperation in a win-win scenario.

Other perspectives: One of the first writers to comment on class struggle in the modern sense of the term was the French revolutionary François Boissel. Other class struggle commentators include Henri de Saint-Simon, Augustin Thierry, François Guizot, François-Auguste Mignet and Adolphe Thiers. The Physiocrats, David Ricardo, and after Marx, Henry George noted the inelastic supply of land and argued that this created certain privileges (economic rent) for landowners. According to the historian Arnold J. Toynbee, stratification along lines of class appears only within civilizations, and furthermore only appears during the process of a civilization's decline while not characterizing the growth phase of a civilization.
Pierre-Joseph Proudhon, in What is Property? (1840) states that "certain classes do not relish investigation into the pretended titles to property, and its fabulous and perhaps scandalous history." While Proudhon saw the solution as the lower classes forming an alternative, solidarity economy centered on cooperatives and self-managed workplaces, which would slowly undermine and replace capitalist class society, the anarchist Mikhail Bakunin, while influenced by Proudhon, insisted that a massive class struggle by the working class, peasantry and poor was essential to the creation of libertarian socialism. 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.