Mexican War of Independence
Although Mexico gained its independence in September 1821, the marking of this historical event did not take hold immediately. The choice of date to celebrate was problematic, because Iturbide, who achieved independence from Spain, was rapidly created Emperor of Mexico. His short-lived reign from 1821–22 ended when he was forced by the military to abdicate. This was a rocky start for the new nation, which made celebrating independence on the anniversary of Iturbide's Army of the Three Guarantees marching into Mexico City in triumph a less than perfect day for those who had opposed him. Celebrations of independence during his reign were marked on September 27. Following his ouster, there were calls to commemorate Mexican independence along the lines that the United States celebrated in grand style its Independence Day on July 4. The creation of a committee of powerful men to mark independence celebrations, the "Junta Patriótica", organized celebrations of both September 16, to commemorate Hidalgo's grito and the start of the independence insurgency, and September 27, to celebrate actual political independence.
Treaty of Guadalupe Hidalgo
Mexico had claimed the area in question since winning its independence from the Spanish Empire in 1821 following the Mexican War of Independence. The Spanish Empire had conquered part of the area from the American Indian tribes over the preceding three centuries, but there remained powerful and independent indigenous nations within that northern region of Mexico. Most of that land was too dry (low rainfall) and too mountainous to support many people, until the advent of new technology after about 1880: means for damming and distributing water from the few rivers to irrigated farmland; the telegraph; the railroad; the telephone; and electrical power.