The United States of Mexico

On March 10, 1848, the US Senate ratified the Treaty of Guadalupe Hidalgo, which formally ended the Mexican-American War.  Depending on your point of view, Mexico either lost about 55% its territory to the US in this War, or the US stole about 55% of Mexico from the Mexicans.  (Image of the United States of Mexico in 1846, from the David Rumsey Map Collection.)