What was the most important historical event of the 1900's?

What was the most important historical event of the 1900's? write a one page essay on the following topic: What was the most important historical event of the 1900's? and include an quotation from a historical source. This is for my US history class (American Civilization). The textbook we are using for this class is Davidson, James West. Experience history: interpreting America's past. New York: McGraw-Hill Higher Education, 7th edition