Short Answer 
The United States significantly contributed to the Allied victory in World War II through troop deployment, a ‘Europe First’ strategy, and post-war financial aid. This approach not only helped defeat Axis powers but also facilitated Europe’s economic recovery through initiatives like the Marshall Plan.
Step 1: Troop Deployment
The United States made a significant impact on the Allied war effort by sending a vast number of troops to Europe. These soldiers participated actively in key battles, helping to secure victories that were crucial for the Allies. Their presence not only boosted morale but also turned the tide against the Axis powers, particularly Germany.
Step 2: ‘Europe First’ Strategy
The United States, along with its allies, adopted a strategic approach known as ‘Europe First’. This meant prioritizing the defeat of enemy forces in Europe, specifically targeting Germany and Italy before addressing the conflict with Japan in the Pacific. This strategy ensured that the majority of resources and focus were on the most immediate threats to peace in Europe.
Step 3: Post-War Financial Aid
In addition to military support, the U.S. played a vital role in the economic rebuilding of Europe after the war through the Marshall Plan. This initiative provided substantial financial aid aimed at restoring war-torn countries, fostering economic stability, and preventing the spread of communism in the region. This long-term support solidified the United States’ commitment to a strong, democratic Europe.