https://en.wikipedia.org/wiki/List_of_Nobel_laureates_by_country#Summary
(USA tops the list, natch)

Also here's a somewhat underappreciated one:
The United States under Woodrow Wilson urging the other countries not to impose harsh war debts on Germany at the Treaty of Versailles after WWI. If he had been successful, he might well have prevented WWII.

Edit: Having researched the matter further, my knowledge of the Treaty of Versailles may be a little dubious. Don't blame me, blame my history teacher.