War began in 1846 over the annexation of Texas and the disputed boundary between the Nueces and the Rio Grande, and ended with the occupation of Mexico City and the signing of the Treaty of Guadalupe Hidalgo on 2 February 1848. Mexico ceded more than 525,000 square miles for fifteen million dollars and the assumption of over three million dollars in claims by American citizens. Mexicans remaining in the ceded territory were promised citizenship and protection of property; in practice land titles in California and New Mexico were widely lost through subsequent litigation and settlement. The acquisition transferred sovereignty over Native nations across the Southwest to the United States and precipitated the political crisis over the extension of slavery into the new territories that ran towards the Civil War.
The United States invaded Mexico and annexed roughly half of its national territory by the Treaty of Guadalupe Hidalgo
Contemporary estimates put roughly 80,000-100,000 Mexican citizens in the ceded lands, the great majority of them in New Mexico, with perhaps 7,500-14,000 Californios and small settlements in Arizona and Colorado. The indigenous population of the same territory was considerably larger and was not counted as Mexican citizens; California alone held an estimated 150,000 Native people in 1848. The territory itself, some 1.36 million square kilometres, was about half of Mexico's national area.
United States military deaths are recorded with reasonable accuracy at about 13,000, the great majority from disease rather than combat. Mexican military and civilian deaths were never systematically counted and are estimated from army returns, hospital records and city accounts; the usual range for the Mexican side is in the tens of thousands. The total given here is therefore an estimate combining a firm figure with a soft one.
Mexico