Westward Expansion and the
Mexican-American War
Unit 6, Lesson 1
Essential Idea
• The ideal of “Manifest Destiny” inspired
Americans to claim the West, which led to the
Mexican-American War.
Manifest Destiny
• Manifest
Destiny:
• Manifest
Destiny—the
idea that
Americans had
a God-given
right to conquer
and civilize
North America
• This idea
encouraged
settlers to move
west before,
during, and
after the Civil
War
Overland Trails
• These settlers,
called
“overlanders,”
used routes
like the
Oregon Trail
• Manifest
Destiny
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Moving West
Reasons for Migration:
1. Economic—some went west for mining, ranching, and farming
“Mountain men” trapped fur and served as guides to settlers
2. Religious—Mormons, led by Brigham Young, went west to escape religious
persecution
Frontier Life
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Life on the Frontier:
Over time, settlers did increasing damage to the environment
As settler population grew, conflict with Native Americans increased
Settlers dealt with isolation, loneliness, a rugged environment, and Native American attacks
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Election of 1844
Election of 1844:
James K. Polk, called the “expansion
president,” ran against Henry Clay
Polk’s platform:
1. Polk was committed to “manifest
destiny” and expansion
2. Polk promised to annex Texas as a
new state
3. Polk promised to acquire
California
4. Polk would end sharing the
Oregon Territory with England
“Fifty-four Forty or Fight!”
• Election
Mandate?
• Polk won,
showing that
Americans
wanted to expand
the country
• Polk’s
Accomplishments
Expansion
Begins
• Texas:
• After Polk’s election, the
United States annexed
Texas
• Consequences:
• Annexing Texas angered
anti-slavery northerners
because it opened new
land to slavery
• Annexing Texas infuriated
Mexico, which felt Texas
was really theirs
• Tension between the
United States and Mexico
rose
Oregon Treaty
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Oregon:
“Fifty-four Forty or Fight!”—this
slogan stated the desire of
Americans to own ALL of the
Oregon Territory up to the 54°
40’ latitude line
However, Polk did not want war
with both Mexico AND England
Oregon Treaty—the United
States agreed to split the Oregon
Territory with England at the 49°
line (not 54°40’)
Consequences:
War with England was avoided
Northerners were upset that
more land for future free states
was not obtained, especially
after Texas had been annexed
Manifest Destiny and Oregon
The MexicanAmerican War:
Causes
• Causes:
• 1. Annexing Texas
• Mexico never fully
recognized Texas
independence
• Many Mexicans felt
that the United States
had “stolen” Texas
• 2. Desire for California
• Polk offered to buy
California (and New
Mexico) from Mexico
• Mexico, angry, refused
to even negotiate,
raising tension
The MexicanAmerican War:
Causes
• 3. Texas Border
Dispute
• Mexico said the
Texas/Mexico border
started at the Nueces
River, the United
States said it started
at the Rio Grande
River
• Both countries
claimed the land
between these two
borders
The Spark of War Ignited
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War Ignites:
Polk ordered troops to march into the disputed territory
The Mexican army attacked and killed 11 of the soldiers
Polk claimed American blood was shed on American soil
Congress declared war on Mexico
Opposition to War
• Opposition to War with
Mexico:
• Many northerners and
abolitionists opposed the war
as a conspiracy to create new
future slave states
• Wilmot Proviso:
• Wilmot Proviso—to appease
northerners, this suggested
making all land won from
Mexico FREE
• Pro-slavery congressmen
made sure the Wilmot
Proviso never passed
The Mexican-American War: The Strategy
• Part 1:
• Capture Santa Fe
in New Mexico
• What Happened:
• Santa Fe was a
trade center for
northern Mexico
• Capturing it cut
much of northern
Mexico from its
southern capital,
Mexico City
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The Mexican American War: The Strategy
Part 2:
Capture California
What Happened:
“Bear Flag Revolt”—
California, like Texas,
was fighting for
independence from
Mexico
American troops
helped free California
Mexico lost control of
its northern half
The Mexican-American War: The Strategy
• Part 3:
• Invade from
North
• What
Happened:
• Troops crossed
the Rio Grande
and invaded
south toward
Mexico City
• Mexican troops
left Mexico City
unguarded to
intercept
American troops
The Mexican-American War: The Strategy
• Part 4:
• Capture Mexico
City
• What Happened:
• General Winfield
Scott took troops
and captured
Mexico City
• With the northern
half of Mexico
gone and the
capital captured,
Mexico had to
surrender
The Mexican-American War Ends
• Terms of Peace:
• Treaty of Guadalupe
Hidalgo—in this
peace treaty, sold its
northern half to the
United States and
recognized the Rio
Grande as the Texas
border
• Polk and Expansion
(Watch first 2:40)
• Mexican Cession—
this land made up
what is now the
American Southwest