U.S
The people of the United States felt it was their mission to extend the "boundaries of freedom" to others by imparting their idealism and belief in democratic institutions to those who were capable of self-government. It excluded those people who were perceived as being incapable of self-government, such as Native American people and those of non-European origin.
To understand Manifest Destiny, it's important to understand the United States' need and desire to expand. The following points illustrate some of the economic, social and political pressures promoting U.S. expansion: • The United States was experiencing a periodic high birth rate and increases in population due to immigration. And because agriculture provided the primary economic structure, large families to work the farms were considered an asset. The U.S. population grew from more than five millon in 1800 to more than 23 million by mid-century. Thus, there was a need to expand into new territories to accommodate this rapid growth. It's estimated that nearly 4,000,000 Americans moved to western territories between 1820 and 1850. • The United States suffered two economic depressions — one in 1818 and a second in 1839. These crises drove some people to seek their living in frontier areas. • Frontier land was inexpensive or, in some cases, free. • Expansion into frontier areas opened opportunities for new commerce and individual self-advancement. • Land ownership was associated with wealth and tied to self-sufficiency, political power and independent "self-rule." • Maritime merchants saw an opportunity to expand and promote new commerce by building West Coast ports leading to increased trade with countries in the Pacific.

Mexico

One of the reasons the U.S/Mexican War. In 1845, with the most of the consent of its citizens, the Republic of Texas was annexed by the United States. This was the underlying cause of the war. The problem was that Mexico objected to annexation, holding that Texas was still part of that country, even though Texans had fought and won their independence nearly ten years earlier and had been formally recognized as a sovereign nation by the United States, Great Britain, France, and other countries. While the United States put into motion a quest for its Manifest Destiny, Mexico faced quite different circumstances as a newly independent country. Mexico achieved its independence from Spain in 1821, but the country suffered terribly from the struggle. The war caused severe economic burdens and recovery was difficult. • Due to Mexico's economic system, there were fewer opportunities for individual self-advancement in the frontier regions and people were less motivated to relocate. Colonization was pushed primarily as part of the government's political agenda. • Constant warfare with Native Americans discouraged people from settling into the areas. •