KEY DATES OF THE UNITED STATES EMPIRE

1803: Thomas Jefferson negotiates the sale of the Louisiana Purchase from the French for fifteen million dollars
1821: The American Colonization Society establishes the colony of Liberia, followed by other efforts by several American states to establish colonies in West Africa
1823: The United States adopts the Monroe Doctrine, which attempts to limit new European expansion into the Americas; in return, the United States agrees not to interfere in European affairs
1836: American settlers in Texas rebel against Mexican rule and create the Republic of Texas
1845: The United States annexes Texas
1846: Disputes over the border between Mexico and Texas lead to the Mexican-American War
1848: The Treaty of Guadalupe Hidalgo ends the Mexican-American War, with Mexico agreeing to give up much of the Southwest and California for fifteen million dollars
1867: Russia sells Alaska to the United States
1893: The United States supports a rebellion in Hawaii
1898: The United States annexes Hawaii, seeing Pearl Harbor as a strategic military base
1898: The Spanish-American War begins in April, though the fighting only lasts until August when the Spanish ask for a truce. The Treaty of Paris formally ends the Spanish-American War, and the United States gains control of the former Spanish colonies of Guam, the Philippines, and Puerto Rico. Cuba is declared independent, but occupied by the United States until 1902
1899: Prominent Americans, including Andrew Carnegie, Mark Twain, and Samuel Gompers, found the Anti-Imperialist League, opposing the expansion of the United States
1901: The U.S. Congress passes the Platt Amendment, making a protectorate of Cuba and retaining the right to intervene militarily in Cuban affairs
1903: The United States supports independence for Panama in exchange for the right to build a canal through the country
1904: President Theodore Roosevelt develops the ‘‘Roosevelt Corollary,’’ suggesting the United States has the obligation to aid smaller countries in the Western Hemisphere when threatened with economic troubles. Using this principle, the United States takes over the finances of the Dominican Republic in 1905, intervenes in Haiti in 1915, and sends the military into Nicaragua on several occasions in the early 1900s
1918: After WWI, the United States advocates decolonization, suggesting that the victorious Central Powers, including France, Great Britain, and Japan, transition their colonies to self-rule
1945: With the end of WWII, the United States looks to sustain influence by creating collations of similarminded democracies, large and small, to balance the Soviet Union’s expanding empire
1950s-1970s: To fight the perceived Soviet Communist threat, the United States covertly replaces leaders in Iran (1954), Guatemala (1954), and Chile (1973) with governments more friendly to the United States

In the book 'Encyclopedia of Western Colonialism since 1450' Thomas Benjamin, editor in chief, Thomsom Gale, Farmington Hills,U.S.A, 2007. p. 435. Edited to be posted  by Leopoldo Costa.