United States in exchange for its independence, it did so in 1845. However, at the time of annexation, the southern border of Texas had still not been specified. US President Polk took the position that recognized the Rio Grande as the southern border. In what was later to be considered a deliberate provocation by the United States to begin a conflict with Mexico, US troops entered the area between the Nueces River and the Rio Grande (land believed to belong to the US through annexation). The conflict that occurred between the US military and Mexican military was considered an act of war by the US, even though Mexico had not confirmed whether or not the Rio Grande was the southern border of the Texas territory.
After many unsuccessful peace negotiations (open and secret) and after many military skirmishes, the US military gained occupation of Mexico City in August 1847. It was then that the final peace negotiations began in what would become the Treaty of Guadalupe Hidalgo. The treaty demanded a large section of Mexico’s northern territory, with the Rio Grande as the southern border (for to accept the Nueces would be admitting guilt for starting the war). It was signed and sent to Mexico and the United States’ senates to be ratified on February 2, 1848.
In the United States, President Polk only conceded to accept it and send it on to the Senate for ratification after coming to the conclusion that continuing the war would not acquire for the United States a treaty that was any better. However, he recommended to Congress that an amended one be ratified and sent to Mexico for approval, one that did not contain Article X, which guaranteed property rights for Mexicans and Indians living in the ceded territory being. His main reason for this recommendation was that questions over the validity of land grants in Texas would come up on whether or not the treaty would apply to Texas since they had acquired their independence prior to the treaty.