briefly explain one historical event or development that caused americans to view the war as a struggle for the survival of democracy against fascism and militarism

The chosen historic event was the entry of the USA into World War II.

Explanation:

As you already know, the second world war was a conflict established, primarily, between European nations. This was a bloody war, marked by the aggressiveness of the German army that was having many victories and advancing in the European territory where it left established a Nazi control, facist, anti-democratic and based on militarism, together with its allies Italy and Japan.

The USA had no intention of entering into this conflict, since participating in a war causes a great loss of life, like a big hole in a country's economy, however, due to the advance and the eminent victory of Germany and its allies and of the great damage that the Japanese army wreaked on Pearl Habor, the American government concludes that participating in the war against Germany, Italy and Japan was a way of protecting the interests of democratic nations, forcing militarism and fascism to lessen their influence.