Mexican Cession Mexican Cession, with considerable local autonomy punctuated by several revolts and few troops sent from central Mexico, in the period from 1821–22 after independence from Spain up through 1846 when U.S. military forces seized control of California and New Mexico on the outbreak of the Mexican–American War. The northern boundary of the 42nd parallel north was set by the Adams–Onís Treaty signed by the United States and Spain in 1821 and ratified by Mexico in 1831. The eastern boundary of the Mexican Cession was the Texas claim at the Rio Grande and extending north from the headwaters of the Jojo
Puebla a commemoration of Mexican culture and heritage, particularly in areas with large Mexican-American populations. Many Americans mistakenly believe that Cinco de Mayo (“May 5th”) is the Mexican equivalent of the United States’ Fourth of July holiday — a date marking the official casting off of colonial rule via the announcement of a new independent country. However, the Mexican version of Independence Day is celebrated on September 16, for it was on that date in 1810 that the commencement of the war for Mexican independence from Spanish rule was pronounced in the small town of Dolores by Miguel Hidalgo y Costilla