As imperialism took hold of European politics, so did Cartesian theory. Existentialism refutes this assurance in the holy purpose of man, and the unquestioning belief in a benevolent God. Ontology, the study of being, is a main concern of all existentialists. Self is nothing but what one's actions make one, which can change at any moment. If human beings are responsible for their own actions, then perhaps human beings possess, too, the power to prevent more deaths and disasters.
Existentialism is the end result of the age of imperialism.
By building empires and setting up alliances like houses of cards, Europe set the stage for the most terrible war the world had yet seen. It was one that involved not only the countries who had grievances with each other, but also allies, and colonies, and nations bound by the many and varied treaties drawn up to avoid the inevitable war. Sartre thought that the Cartesian view of the universe as man's toy had taken the world to this point of chemical warfare and mass death. He held that human life should be viewed by its actions, not its thoughts. Further, humanity shouldn't lean on the prop of religion to solve their problems.