James K. Polk was President during the Mexican American War.
The Mexican American War begin with the dispute between Mexico and the United States over a border along the Rio Grande. James K. Polk was President at the time and wanted what is today the U.S. southwest to be part of the United States. He tried to purchase the land from Mexico, but when his offer was denied, he put troops into a disputed zone between the Rio Grande and Nueces River. It was on April 25, 1846 when the war actually began. A group of US soldiers under the command of General Zachary Taylor were attacked by Mexican cavalry. About a dozen were killed. A siege was then laid on an American fort along the Rio Grande. The US troops managed to defeat the Mexicans due to reinforcements and their more-advanced artillery in the battles of Palo Alto and Resaca de la Palma. American then declared war on Mexico after those two battles on May 13, 1846. Since only 75,000 Mexican citizens inhabited land north of Rio Grande, it did not take much from Col. Stephen W. Kearny and Commodore Robert F. Stockton to conquer those lands. The same went for General Taylor and he captured Monterrey. Mexico was suffering loss after loss. This was when they called up General Antonio Lopez de Santa Anna. Santa Anna at the time was living in exile in Cuba. When he came back to Mexico, he took took command of the Mexican troops and led them into battle with the US. He was forced to withdraw from battle during during the Battle of Buena Vista in February, 1847. General Winfield Scott along with US troops invaded and conquered Veracruz. Later in September 1847, he captured Mexico's City Chapultepec Castle. The End of the War The Mexican American War ended with the surrender of Mexico. A agreement was to be made between the two nations. On February 2, 1848, the Treaty of Guadalupe Hidalgo was signed, confirming that the Rio Grande was the border between Mexico and the United States. The treaty also recognized Texas' annexation and agree to sell the territory of California that was north of the border to America for $15 million dollars.