1. not many settlers where there and those that were living in those areas were neglected by the Mexican government. 2. on may 12,1846, the united states senate voted 40 to 2 to go to war with Mexico. president james k.polk had accused Mexican troops of having attacked Americans on u.s. soil, north of the rio grande. but Mexico claimed this land as its own territory and accused the Americans military of having invaded. 3. in early 1848, both the us and Mexico signed the treaty of Guadalupe Hidalgo. under this agreement, Mexico gave up Texas and a cast region called the Mexican cession. the return, the united states agreed to pay Mexico $15 million. it also promised to protect the 80,000 to 100,000 Mexicans living in Texas and the Mexican cession. but, most of these promises were not kept. some senators argued that the US had no right to any Mexican territory other than Texas.some argued that new Mexican and cali even combined were worthless. other senator opposed that treaty because they wanted more land.
The fourteenth amendment says "All persons born or naturalized in the Unites States, and subject to the juisdiction thereof, are citizens of the United States"
