The main initiative that the US took in the 1920s to help ensure economic stability and peace in Europe was to bring into effect the "League of Nations," although the US ultimately ended up not joining this group.
It would be "germs and disease" that had the greatest impact when the Old and New Worlds met, since the Natives in the New World were not immune to the germs that the Europeans were carrying--leading thousands of Natives to be killed.
