Answer:
World War 1
Step-by-step explanation:
Following the World War 1, in memory of the great losses in WW1, combined with the previous stances of isolationism throughout the 19th century, many in the US advocated non-involvement in international affairs. This is clearly see in the United States not joining the League of Nations, due to this isolationist stance. Furthermore, the consequent years after WW1, the 1920's, was a time of great prosperity, as during the War the United States transitioned from a debtor to a creditor nation, aswell as huge increases of GNP as well as GDP.
Therefore, World War 1 is the correct answer for the blank.