In the early 1840s, the United States pushed for expansion westward after buying much of the land.  This push for expansion was primarily for new opportunities and especially money! This idea for expansion and wealth was labeled as Manifest Destiny.

As for the Mexican-American War, the battle of Buena Vista was sparked by the United States ' original annexation of Texas and the ongoing conflict with the local Mexican's. At the time, the United States didn't want to make Texas its own state with the rest of the U.S. because it would disturb the equal balance of slave states vs non-slave states.  At this time, the current president, James K. Polk, wanted to purchase California from Mexico, but they refused.  At this point, President Polk was trying to find any reason to start a war with Mexico, so he sent troops across the border to get Mexican troops to shoot them, and it worked, starting the official war.

As for the Battle of Buena Vista, it was fought near the end of the Mexican-American War and was a major conflict in deciding who won the war.  It was fought in north-eastern Mexico in modern-day Colorado. Santa Anna decided to ambush General Taylor, but Taylor had already moved his troops to stay at La Angostura near the hacienda (city or estates) of Buena Vista between two high ranges of mountains on February 21st.  Mexican troops cut off Taylor's communications, and on the 23rd of February, the real battle began.