1) In the late 19th century, many American leaders felt the U.S. should establish overseas colonies like European nations. This began America's period of imperialism.
2) The U.S. acquired Alaska from Russia in 1867 and annexed Hawaii in 1898, seeing economic and strategic value in the territories.
3) In 1898, the U.S. went to war with Spain over Cuba's struggle for independence. The U.S. victory resulted in it gaining control of Puerto Rico, Guam, and the Philippines. This marked America's emergence as a world power.