Answer:
Historyguy.com:
The U.S.-Mexican War (1846-1848)
The U.S.-Mexican War—(1846-1848): The Mexican-American War was the first major conflict (continue reading..)
Mexican War
U.S. Troops at the Siege of Veracruz
The U.S.-Mexican War—(1846-1848): CAUSES, TIMELINE, AND FACTS
The Mexican-American War was the first major conflict driven by the idea of "Manifest Destiny"; the belief that America had a God-given right, or destiny, to expand the country's borders from 'sea to shining sea'. This belief would eventually cause a great deal of suffering for many Mexicans, Native Americans and United States citizens. Following the earlier Texas War of Independence from Mexico, tensions between the two largest independent nations on the North American continent grew as Texas eventually became a U.S. state. Disputes over the border lines sparked military confrontation, helped by the fact that President Polk eagerly sought a war in order to seize large tracts of land from Mexico.
CAUSES OF CONFLICT: Why Did the Mexican-American War Start?
The war between the United States and Mexico had two basic causes. First, the desire of the U.S. to expand across the North American continent to the Pacific Ocean caused conflict with all of its neighbors; from the British in Canada and Oregon to the Mexicans in the southwest and, of course, with the Native Americans. Ever since President Jefferson's acquisition of the Louisiana Territory in 1803, Americans migrated westward in ever increasing numbers, often into lands not belonging to the United States. By the time President Polk came to office in 1845, an idea called "Manifest Destiny" had taken root among the American people, and the new occupant of the White House was a firm believer in the idea of expansion. The belief that the U.S. basically had a God-given right to occupy and "civilize" the whole continent gained favor as more and more Americans settled the western lands. The fact that most of those areas already had people living upon them was usually ignored, with the attitude that democratic English-speaking America, with its high ideals and Protestant Christian ethics, would do a better job of running things than the Native Americans or Spanish-speaking Catholic Mexicans. Manifest Destiny did not necessarily call for violent expansion. In both 1835 and 1845, the United States offered to purchase California from Mexico, for $5 million and $25 million, respectively. The Mexican government refused the opportunity to sell half of its country to Mexico's most dangerous neighbor.
