Herbert Hoover believed that the government should not intervene in the affairs of American citizens because they got themselves into their own financial mess so they needed to get themselves out of it. This theory was called rugged individualism.
This is because it was when the Allies were able to defeat the Axis powers, which gave hope to the Allies to fight stronger. Also, when they defeated Axis powers at that battle, they(ALLIES), now had vantage points on both sides of Germany; east(UK) and west (Russia).