technically began in 1846 over disputes over the Texas border (Rio Grande vs. Nueces River), but it can be argued that the annexation of Texas was the beginning of diplomatic trouble with Mexico. This finally boiled over when the Mexican Army crossed the Rio Grande and captured the American army patrol on what the Americans thought was their land (ironically, the Mexicans thought that was their land). Also, it erupted because the U.S. wanted to buy California, but Mexico refused to give it to them. In the end, Mexico lost and U.S. gained the Mexican Cession, California, and established the border separating the two countries at the Rio Grande River. James K. Polk was president during this