After a brief period of recession, post-war America from 1921 onwards until 1929 grew to become one of the most economically powerful superpowers that the world had ever seen at the time. The prosperity from war marketing meant that not only had America advanced its own economic strength dramatically, but it's incredibly late entry into the war meant that large parts of Europe were in financial peril after being heavily damaged by the war. This worked as a basis for America to spend the next 8-10 years moving through a period of snowballing growth where industry, innovation & capitalism were allowed to boom in a way the world had never quite experienced before.
The US did not join the war until 1917; 3 years after the war had started, however they had not made little of their time. Although America adopted an isolationist policy which frowned upon intervention in international conflict, they had instead spent the last 3 years taking over global industries and strengthening domestic economies by selling arms and products to countries that could not manufacture them on their own as a result of the war. They did not pick a side of the war effort, nor did they necessarily cater to those who tried to buy them as an ally, however they did provide to both sides for a large part of the war. Although they did enter the war in 1917, it was seen as more of a 'clean up' effort and didn't place too much of a drain on resources or national industry. When the war ended in 1919, a brief period of economic downturn began as a consequence of the American involvement, however it did not last long. .
In 1921, the Republican Party – and Woodrow Wilson – were voted out of office in favour of the republican Warren Harding. Harding did not stay in office long; however his brief stint in the white house began the economic turnaround that lead to the huge 8 year boom in the 1920s. Harding died on office, and was succeeded by Calvin Coolidge, a main who firmly believed in 'Laissez-Faire' economics; the idea that every individual should build be free to trade amongst themselves without fear of government intervention.