Answer:
The most significant catalyst for the development of Existentialism was World War I.
Step-by-step explanation:
The devastation and trauma of the war led many people to question traditional beliefs and values, and to confront the existential themes of meaninglessness, alienation, and the search for personal identity. Existentialism emerged as a philosophical and literary movement in the aftermath of the war, with influential thinkers such as Jean-Paul Sartre and Martin Heidegger exploring these themes in their work. The fall of the Roman Empire, the American and French revolutions, and the Great Depression were important historical events, but they did not directly contribute to the development of Existentialism in the same way that World War I did.