To what is The Communist Manifesto a reaction?
For Marx and Engels, the widening condition of capitalism is what necessitated the writing of The Communist Manifesto. Both thinkers are aware that the expansion of capitalism and its prevalence in so much of social, economic, and political affairs demanded a reaction. Marx and Engels put forth the argument that capitalism is the latest in a historical dynamic of dialectical materialism that reduces human interaction to a state of those who own the means of production and those who are relegated to the outside of such power. Capitalism is the latest installment in this dialectic.
The conditions of capitalism contain specific realities in which specific reactions must be articulated. The treatment of workers in a capitalist system is one such reality in which a reaction was merited. The exploitation of workers who do most of the work and receive a paltry amount of the reward is one such condition. Marx and Engels observe the exploitation of the worker at the hands of the bourgeoisie as something that demands a reaction. At the same time, Marx and Engels believes capitalism to be fundamentally undemocratic. This state of being is one in which the few (those in the position of power of wealth) control the vast majority (those who lack the ownership in the means of production.) For Marx and Engels, this state of being is one in which a reaction in the form of articulating the new vision of socialism is required.