The primary causes of the Mexican-American War were mainfest destiny, westward expansion, economics, and slavery. Throughout the nineteenth century, Americans believed in manifest destiny, asserting that it was the United States' right to expand westward and conquer territory despite the rights of the indigenous people already residing on the land. In the election of 1844, presidential nominee James K. Polk promised to re-annex Texas if he was elected President. Following the campaign, newly elected President Polk adopted the same policy of his predecessor, John Tyler, and aimed to annex Texas and other territories in the West. In addition, several American citizens previously settled in Texas since they believed the United States would eventually annex the territory. By settling in Texas, these individuals planned to sell their land at higher prices to Americans once the territory became part of the United States. Additionly, the settlers were able to profit from trade opportunities in the West. Another cause of the war was slavery. American citizens in the south wished to gain more "slave states" in order to increase their political power.
Prior to the Mexican-American War, Mexico suffered from many internal difficulties. In 1821, Mexico gained independence from Spain; however, the struggle caused severe economic hardships. In 1824, Mexicans replaced their monarchy with a constitutional republic, which created internal political struggles between different factions. Since Mexico was preoccupied with internal economic and political challenges, the nation focused little attention on the northern territories acquired from the war with Spain. As a result, tensions arose between the central government and frontier communities. It is clear that all of these challenges caused Texas to acquire independence, which exacerbated the relations between Mexico and the United States.