They insisted that their nation had a manifest destiny to dominate the continent and felt that it was their mission to extend the “boundaries of freedom” to others by passing on their idealism and belief in democratic institutions to those who were capable of self-government. Firstly, during that period of time, the United States was experiencing high population growth rate from increasing in birth rate and immigration. Therefore there was a need to expand into new territories to accommodate with this huge population growth. Secondly, the United States suffered from two economic depressions therefore driving people to seek living in frontier areas since frontier land was inexpensive or free. Thirdly, expansion into frontier areas provided new opportunities for commerce and individual self advancement thus promoting economic growth. The Mexican American War was purely driven by economical, social and political pressures to expand U.S