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Westward Expansion, 1790­1850
THE UNITED STATES DID NOT HAVE A LEGITIMATE CLAIM
The United States was not justified in its actions against Mexico during the Mexican­American War. Mexico's legitimate
historical claims to the ceded Texas land and to California stem from two key factors—first, that the United States officially
recognized the territorial boundaries of Mexico before that issue was clouded by the Texas Revolution of 1835–1836; and,
second, that the primary cause of the Mexican­American War was American hunger for Mexican land. Treaty of Guadalupe Hidalgo
Treaty of Guadalupe Hidalgo
With the ratification of the Treaty of Guadalupe Hidalgo by the Mexican and U.S. senates in 1848, the Mexican­American
War ended and well over 500,000 square miles of territory officially passed into American hands. This agreement is the oldest still in force between the two
nations. It is also the most controversial, especially in Mexico. The reason is clear—the victors of the war dictated the terms of an accord that was designed to
serve American interests. Although Mexico retained a legitimate, historical claim to Texas and California, the treaty recognized the military situation on the ground, one in which American
forces occupied most of the land claimed and, indeed, much of Mexico that was not. Mexico was forced to relinquish any claim to Texas, whose independence
and later annexation by the United States the Mexican government had until now refused to officially recognize. Mexico also surrendered title to land west all
the way to the Pacific Ocean, including the future state of California. Parallel History and Discord
During their early history, Mexico and the United States traveled similar paths. Both based their claims to territorial sovereignty upon imperial pasts. Both broke
away from their mother countries after successful revolutions and established republics (Mexico achieved independence in 1821 and established a republic in
1824 after overthrowing Emperor Agustín de Iturbide). Early on, both understood each other as fellow travelers along a common republican path. Mexico's
1824 constitution, for example, intentionally bore many similarities to that of the United States. Mexico and the United States also shared a well­defined border.
In 1819, the Adams­Onís Treaty between the United States and Spain established the border along the Sabine River. Furthermore, the United States
relinquished any claim to Texas in Article III of the same treaty. Mexico inherited both the boundary and the claim to Texas when the United States became one
of the first countries to recognize Mexican independence in 1822. Expansionism, a perpetual theme in antebellum American history, eventually overwhelmed the incipient republican amenity between the two countries as well as
the border. Spanish colonial authorities had long eyed their northeastern neighbor's expansionist tendencies with suspicion. Thus, Joel Poinsett, the first U.S.
ambassador to a newly independent Mexico, surprised no one when he advised the Mexican government of American desire for Texas, California, and New
Mexico. Thousands of Americans moved across the Sabine during the 1820s and early 1830s. Most accepted Mexican citizenship. Later, these same
immigrants would fight for their independence against their erstwhile countrymen.
Antonio López de Santa Anna
The Texas Revolution proved popular in the United States, which made it clear to many Mexicans that land hunger had
trumped republican sympathy and respect for the border in American hearts and minds. Mexico quickly rejected the Treaties of Velasco, which were
"negotiated" by Antonio López de Santa Anna after his capture, and recognized Texan independence and set the Lone Star Republic's border upon the Rio
Grande. The United States, on the other hand, recognized Texan independence on March 3, 1836. Predictably, Mexican­American relations gradually soured
until the year 1845 saw them come to a head with the annexation of Texas and the inauguration of James K. Polk as president of the United States.
Texas and the War
President Polk's small physical stature was no reflection of the size of his expansionist plans. Indeed, Polk, who styled himself an agent of manifest destiny,
planned on enlarging the country by half by adding Oregon, Texas, and California to its domain. John Tyler, the previous president, took care of Texas by joint
resolution of Congress in 1845, and an agreement had been worked out with Great Britain over the Oregon Territory by early 1846. This left California and
other Mexican territory to the east. Americans had long looked at this land with covetous eyes, particularly the superb port of San Francisco. Polk considered
several methods of satisfying his and his countrymen's desire for California, the most notable of which was John Slidell's 1845–1846 diplomatic mission to
Mexico. Slidell's instructions were simple—offer money for land. When Mexico failed to deal, the military option took center stage. The annexation of Texas and the consequent dispute over its border with Mexico provided the pretext for armed confrontation. Mexico considered Texas as
sovereign territory and broke off diplomatic relations with the United States. As half of the U.S. Army marched to Corpus Christi to "protect" the new state, a
Mexican army gathered at Matamoros 150 miles to the south. Between the two armies rested what historians now and many people then referred to as
"disputed territory." Texans claimed the Rio Grande as their state's border, although it had never served as a provincial boundary nor was it ever "controlled"
by the Lone Star Republic. In Mexico, even those willing to accept the loss of Texas would only concede the Nueces River as its border. Polk then made a
fateful decision; on January 13, 1846, he ordered an American army to march south. This aggressive move made war a probability. Diplomacy, the president
understood, offered little hope of achieving his aims regarding California. War, Polk told his cabinet in April 1846, was the answer to the Mexico question. And
so it proved to be.
About the Author
Gregory S. Hospodor
Gregory S. Hospodor earned his BA in history from the College of William and Mary, his MA in history from the University of Mississippi, and his PhD in
history from Louisiana State University, where Charles Royster served as his adviser. He has presented scholarly papers and published on a wide range of
so it proved to be.
About the Author
Gregory S. Hospodor
Gregory S. Hospodor earned his BA in history from the College of William and Mary, his MA in history from the University of Mississippi, and his PhD in
history from Louisiana State University, where Charles Royster served as his adviser. He has presented scholarly papers and published on a wide range of
topics related to the Mexican­American War, most recently a chapter on the home front in Daily Lives of Civilians in Wartime Early America: From the Colonial
Era to the Civil War (2007). Presently, he serves as assistant professor of military history at the U.S. Army Command and General Staff College, Fort
Leavenworth, Kansas.
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Image Credits
Antonio López de Santa Anna: Library of Congress
Treaty of Guadalupe Hidalgo: Library of Congress
MLA Citation
Hospodor, Gregory S. "Manifest Destiny and Mexico: the United States Did Not Have a Legitimate Claim." American History, ABC­CLIO, 2016,
americanhistory.abc­clio.com/Topics/Display/1192400?cid=13. Accessed 7 Dec. 2016.
https://americanhistory.abc­clio.com/Topics/Display/1192400?cid=13
Entry ID: 1192404