World War 2 was a major conflict that resulted in many deaths and can also be called the Second World War.
After the First World War, the League of Nations was established to unify the countries and, in theory, prevent such an aggressive calamity from occurring again.
After the Second World War, this was supplanted by the United Nations. The headquarters is located in New York City of the United States. This, of course, would expand our abilities to influence world affairs.
Before the First World War, the United States attempted to practice isolationism. After the Second World War, as the Russians occupied Eastern Europe, and the United States was allied with Western Europe, an intense Cold War evolved. This was after the United States used the atomic bomb against Japan. Russia and the United States would intensely compete for nuclear weapons prowess.