"The war grew out of unresolved conflicts between Mexico and Texas. After having won its independence from Mexico in 1836, the Republic of Texas was annexed by the United States in 1845; however, the southern and western borders of Texas remained disputed during the Republic's lifetime. That same year tensions between the two countries over territory were raised when the United States government offered to pay off the Mexican debt to American settlers if Mexico allowed the U.S. to purchase the territories of Alta California and Nuevo México from Mexico."
Copied from: http://www.sheppardsoftware.com/Mexicoweb/factfile/Unique-facts-Mexico10.htm
Copied from: http://www.sheppardsoftware.com/Mexicoweb/factfile/Unique-facts-Mexico10.htm