Ronald Reagan attained the rank of captain while serving in the army.
Answer:
On June 5, General Dwight D. Eisenhower, the Supreme Allied Commander in charge of Operation Overlord, decided that the invasion would happen the next day, in part because the weather was still rough and Nazi planes were grounded.
Mainly the events of WWII. Postwar, European powers including Russia was devastated, and the US had a booming economy and prime military power including the atomic bomb. As a result the US became more involved in the world to ensure lasting peace. Individuals would include Pres. FDR, Truman, politicians of the time etc.