Probably the most important reason that the United States became a world power after 1865 was that it became an industrial power. For many imperialists at the time, this offered a powerful incentive to expand American influence—the desire to gain and protect markets for American industry around the world. By the end of the nineteenth century, the United States was among the world's leading economic and industrial powers, so it sought to become an imperial power as well. The most important turning point in this process was the Spanish-American War. This brief conflict, which took place in 1898, ended with the United States in possession of multiple territories formerly belonging to Spain, including the Philippines, Cuba, Puerto Rico, and Guam. Due to promises made before the war, Cuba became independent (if only as a US protectorate), but the other territories became part of an American empire. Other important turning points in American imperialism include the following:

• The purchase of Alaska in 1867.
• The annexation of Hawaii in 1898.
• The "Open Door Notes" by Secretary of State John Hay asserting that the United States should have equal access to markets in China.