1. Notes on the period:
Because there had been a TRANSPORTATION REVOLUTION and a MARKET REVOLUTION, Americans were able to flood the “west” with settlement, using the “Overland Trails”…
Because Americans flooded the West, they initiated conflict with Indians and Mexicans
Because Americans favored expansion, they tended to defend it with themes like “Manifest Destiny”: see John Gast’s “American Progress”:
And because Mexico resisted American aggression, there was a MEXICAN-AMERICAN WAR …

2. WHY was there a Mexican War?
Mexico had secured independence from Spain by 1821, but operated as a rather loose union … by the early 1830s thousands of American settlers, many of them slaveholders, had settled in what is now Texas—and would eventually pursue independence.
Texas secured its independence (“Remember the Alamo”?) with the help of Tejanos, Spanish-speakers born in Texas, in 1836 and would be independent until 1845 when the United States moved to annex it. When the U.S., in defiance of Mexican wishes, moved to annex Texas, war was imminent.
Was it “one of the most unjust ever waged by a stronger against a weaker nation”? (then-Lt. U.S. Grant)

3. WHY a Mexican War, II?
President James K. Polk had campaigned on a platform that focused on territorial expansion—the reoccupation and control of the Oregon Territory and the annexation of Texas, which had been occupied by thousands of cotton-growing Southerners …
Congress adopted a joint resolution in favor of Texas annexation in 1845, causing Mexico to sever diplomatic relations with the U.S. … and President Polk to send troops, led by General Zachary Taylor, into disputed Texas territory.
By 1846 controversial skirmishes led Polk to notify Congress that “war exists”—it complied, and authorized a 50,000-solider increase of the army …

4. What happened in the Mexican War?
American forces overwhelmed those of Mexico during the two years of war …
Abraham Lincoln, a freshman Whig Congressman from Illinois, protested President Polk’s actions, asking to know the spot on which American blood was shed
The Treaty of Guadalupe Hidalgo saw 500,000 miles of territory change hands, an amount rivaling the LA Purchase … Congress appropriates over $18 million in compensation for Mexico

5. What changed because of the Mexican War?
California and what would be, in just a few years, $200 million worth of gold
Future states of Nevada, Utah, New Mexico, Arizona, parts of Wyoming and Colorado, and a no-longer independent Texas
An intensified competition for new land by settlers—and intensified political squabbling over whether new states would be slave, or free …
Practical experience for a generation of soldiers …
(some facts and details from Don’t Know Much About History, by Kenneth Davis)

6. California, Gold, and Conflict:
Because California was American and yielded gold, settlers “rushed” to settle it, causing San Francisco to go from 1,000 to 35,000 people in two years …
Because CA boomed so quickly that it qualified almost immediately for statehood, the Union faced a repeat of the MO dilemma—with no slave state ready to balance it …
Because Southerners could not rely on a state for balance, they held out for a compromise regarding CA’s admission:
THE COMPROMISE OF 1850