Imperialism: The policy or practice by which a country extends its power through acquiring territories outside its borders.
World War I: A global conflict that lasted from 1914 to 1918 involving many nations around the world. It was one of the major events that spurred America's increased involvement internationally.
League of Nations: An international organization founded after World War I with the aim of promoting peace and resolving conflicts. It was an early example of America's commitment to global cooperation.