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
Decolonization of the Americas further wars were fought, such as the Paraguayan War and the War of the Pacific. A related process took place in what is now Mexico, Central America, and parts of North America between 1810 and 1821 with the Mexican War of Independence. Independence was achieved in 1821 by a coalition uniting under Agustín de Iturbide and the Army of the Three Guarantees. Unity was maintained for a short period under the First Mexican Empire, but within a decade the region fought against the United States over the border lands (losing bordering lands of California and Texas). Most of the heat