The Mexican-American war determined the destiny of the United States of America; it determined if there was going to be expansion. The war started mainly because of the Manifest Destiny, Americans thought they had the divine right to extend their territory. The Mexican-American War started mainly because of the annexation of the Republic of Texas (established in 1836 after breaking away from Mexico). The United States and Mexico still had conflicts on what the borders of Texas were. The United States claimed that the Texas border with Mexico was the Rio Grande, but the Mexicans said that it was the Nueces River, so the land in between was disputed and claimed by both the United States and Mexico.
Conflicts started on April 24th, 1846, 2000 Mexican cavalry crossed the Rio Grande and attacked American soldiers." Polk appeared before Congress on May 11 and declared that Mexico had invaded the United States and had ["SHED AMERICAN BLOOD ON AMERICAN SOIL!"]" (ushistory.org,1). After this the United States declared war on Mexico. The Americans used a new artillery method called flying artillery, in which mobile light artillery was mounted on horse carriages, in addition, the shells exploded on impact. The American Artillery demoralized the Mexicans, "The violence of the war and its unjust nature was very traumatic to Mexican pride and is still a source of resentment for Mexicans today which can be seen in the Reconquista movement in Mexico to recover the lost Mexican territories.", (Mexicahistory.org).During the battle of Resaca de la Palma, both sides engaged in vicious hand to hand fighting, the American Cavalry managed to capture the Mexican Artillery resulting in the Mexicans retreating and rerouting, but because of the terrain, it was difficult to rally Mexican troops away. The Mexicans had heavy casualties and were forced to abandon their artillery and other supplies.