If there is a single thesis statement in the Communist Manifesto, it is this, found in the introduction to the document:
[N]ot only has the bourgeoisie forged the weapons that bring death to itself; it has also called into existence the men who are to wield those weapons — the modern working class — the proletarians.
This statement sums up the core arguments of the Manifesto: history is properly understood in terms of class relations based on economic terms; conflict between social classes advances history; industrial development has created the bourgeoisie and the proletariat; and history is moving inevitably toward the destruction of the former by the latter.
The context for the Manifesto was the revolutions that convulsed Europe in 1848—revolutions which Marx and Engels saw as harbingers of the open class confrontation that would destroy the bourgeoisie. By this point, the authors argued, the bourgeoisie had become so powerful and had so thoroughly ripped apart the traditional forces that bound society that a cataclysm was near.
To put it another way, only by increasing exploitation of the proletarian working class could the bourgeoisie survive. The more the working class was exploited, however, the more restive it would become, and the more alienated it would be from the fruits (the economic value) of its labor. In other words, the more successful the bourgeoisie was, the more dangerous and disaffected—not to mention numerically stronger—the proletariat would become. The fact that the proletariats would be increasingly crowded into factories, instead of piecemeal across farms and workshops, would make them easier to organize and more likely to understand their own class interest.
In this way, as the Manifesto argued, the bourgeoisie's expansion and success paved the way for its inevitable destruction. There are many memorable passages in this extraordinary document, but this one encapsulates its thesis.