Mexican-American war is considered to be very important event in the history of the United States. After the end of this war US enlarge their territory a lot. Consequences of this war further led to the Civil War. James Knox Polk, the eleventh President of the United States, had several reasons to start the war with Mexican government. First of all, in that time California belonged to Mexico. It was a lucrative piece of land; basically because it allowed American ships to stay at San Francisco Bay and this in turn allowed United States to trade actively wit Asia. James Polk first decided to buy California but the negotiations failed and unwillingness of Mexicans to cooperate was considered as a threat to American nation.
Secondly, Texas proclaimed its independency from Mexico in 1836. It was recognized as a state by several countries including the United States, France and the United Kingdom. But Mexico refused to do it. Of course, this generated a lot of tensions between two neighbored countries. Furthermore, the US government offered Texas to join the United States. Mexicans considered this step as a threat of the war. James Polk even sent General Zachary Taylor and his army to defend Texas in case of the armed conflict. This means that the US President first of all wanted to acquire new territories and therefore strengthen the power of the United States in Northern America.
The war in Mexico was widely discussed because it had several important opponents. One group was represented by New England abolitionists or anti-slave movement, who could not accept adding a slave state, Texas, to the Union. Besides, the Party of Whigs and one of its leaders Abraham Lincoln also opposed the war actions. According to the statement of the Party, it was more rational to improve industrial potential of the country rather than expand its territory. Besides, the party also did not support the expansion of slavery in the Union.
Platcher, M. James K. Polk and the US Mexican War: a Policy Appraisal. Retrieved from: http://www.pbs.org/kera/usmexicanwar/prelude/jp_jp_and_the_mexican_war.html