The class struggle is the conflict that develops within every mode of production based on exploitation and therefore on class division. It informs all the ideological expressions of an era, shapes the state and drives its transformation or collapse.
Historical function of the class struggle
The class struggle materializes in social terms, that is, it "subjectivizes", it endows with a subject, the conflicts that permanently arise between the productive forces of a society and the social relations within which they develop.
Freeman and slave, patrician and plebeian, lord and serf, guild-master and journeyman, in a word, oppressor and oppressed, stood in constant opposition to one another, carried on an uninterrupted, now hidden, now open fight, a fight that each time ended, either in a revolutionary reconstitution of society at large, or in the common ruin of the contending classes.
Karl Marx and Frederick Engels. Manifesto of the Communist Party, 1848
These struggles are not occasional or random; they are intrinsic to every class system and therefore accompany it from its origin until it is overcome. The oppressed classes will continuously try to shake off the yoke of their oppressors; the exploited, to free themselves from exploitation.
All history (ever since the dissolution of the primaeval communal ownership of land) has been a history of class struggles, of struggles between exploited and exploiting, between dominated and dominating classes at various stages of social evolution.
Frederick Engels. Preface to the German edition of the Communist Manifesto, 1883
But although all the exploited classes have always fought against their situation, the insufficient development of society's productive forces (the material impossibility of abundance, of communism) reserved the role of revolutionary class for other classes. Only the coloni at the end of the slave era or the bourgeoisie under the feudal regime could, in their historical context, transform the juridical-political structure of society according to their particular interests while at the same time producing a progressive alternative for society as a whole.
With the coming to power of the bourgeoisie and the development of the capitalist mode of production, however, the exploited class and the revolutionary class coincide for the first time.
This struggle, however, has now reached a stage where the exploited and oppressed class (the proletariat) can no longer emancipate itself from the class which exploits and oppresses it (the bourgeoisie), without at the same time forever freeing the whole of society from exploitation, oppression and class struggles.
Frederick Engels. Preface to the German edition of the Communist Manifesto, 1883