a war (1846-48) fought between the US and Mexico. It began after Texas became a state in 1845 and there was a dispute about where the border between Texas and Mexico should be. The war was won by the US forces under General Zachary Taylor, who became President a year later. The Treaty of Guadalupe Hidalgo ended the war, and the US paid Mexico $15 million for land that is now in California, Arizona, Colorado, Nevada, New Mexico and Utah. Many people in the northern states worried that the victory would increase the number of the states with slaves, and this worry was one of the causes of the Civil War.