The Beginning of the Mexican-American War 1845-1847

By Allie Reed ’13

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Although the cause of the Mexican-American War (1846-1848) was rooted in American expansionism, the war was ultimately caused by the desire to protect the interests of the United States in California. Tensions had developed between the United States and Mexico as a result of the American effort to settle lands under Mexican control, although the breaking point was the annexation of Texas, a former Mexican territory.  The Americans ultimately won the war, and through the Treaty of Guadalupe-Hidalgo gained over 500,000 square miles of territory, including present-day California, Arizona, and New Mexico, as well as parts of Utah, Colorado, Nevada and Wyoming.[1] This map focuses on the first part of the war, leading up to the Treaty of Cahuenga, which ended the conflict in California.

The major events represented in the map had a significant impact on the diplomatic policy of the United States during the 1840s. Influenced by policies such as the Monroe Doctrine and the notion of Manifest Destiny, the United States began to acquire territories as expansionist tendencies began to take hold. This acquisition of territory was a major development in the growth of the United States. However, it can be asserted that the Mexican-American War was not entirely necessary. Texas had already been annexed and Mexico was already weak. Walter McDougall argues that Polk provoked the war in 1846 in order to focus on securing California. While California promised “one of the finest, if not the very best harbor in the world,” the desire to acquire California was just as much about security as it was about economics.[2] If the British or the French had acquired California, it would have threatened American security and liberty at home. So while the desire to expand the borders of the United States was surely a driving force in the Mexican-American War, the war was mostly about protecting American interests in California.

The map depicts the major battles and events that occurred during the conflict in California up to the signing of the Treaty of Cahuenga in 1847. Each battle is represented by the flag of the country that won; the map shows mostly American flags, as the Americans won many of the major battles. In addition to focusing on the events during the California campaign, the map describes the initial events that sparked the Mexican-American War, including the annexation of Texas, the Thornton Affair, the Siege of Fort Texas, and the Battle of Resaca de la Palma. These were the first few significant events of the Mexican-American War and one must understand how the war officially started in order to analyze the underlying causes of the war.

The map is organized chronologically in order of when the events occurred.  Although tensions between the two countries had been building for years, the annexation of Texas pushed the Mexicans over the edge. As referred to in the map, Mexico had initially invited Americans to settle in northern Texas but “after a decade it became plain that the Americans resented Mexican rule.”[3] After Texas declared independence in 1836, the Mexicans warned the United States that annexing Texas would lead to war. After President James K. Polk was elected on a pro-annexation platform, Congress passed a Joint Resolution that officially made Texas a part of the United States on December 29, 1845 (referred to in the place marker). Mexico “responded by breaking off diplomatic relations.”[4] The place marker includes a map of the Texas boundaries at the time of annexation.

Although the annexation of Texas was the driving force behind the start of the war, the real impetus was the Thornton Affair on April 25, 1846. This was the first use of force during this conflict; the Mexicans ambushed a group of American soldiers led by Captain Seth Thornton (pictured in the place marker) and fourteen men were killed. This led to an immediate declaration of war by the United States on May 13, 1846. In his speech to Congress, President Polk used the fact that “American blood has been shed on American soil” as a call to arms.[5] It can be argued that Polk used this attack in order to justify going to war for California. Like so many other events in history, the Thornton Affair was exactly what the United States needed in order to declare war on Mexico.

The first real battle of the war was the Siege of Fort Texas on May 3, 1846, which occurred before war had officially been declared. As soon as General Taylor left the fort, Mexican forces attacked. The small fort was faced with constant bombardment for six days until the American army came to relieve them. The Mexicans suffered heavy casualties and “since they were not driven from the field, the Americans claimed a victory, although it was not a decisive one.”[6] The place marker includes a link to the official report of the battle. The day after the battle following the Siege of Fort Texas, there was a battle at Resaca de la Palma. “This time both infantry and dragoons were called into play” and both sides suffered large losses.[7] The Mexican army was eventually driven back, marking the second American success of the war. The place marker includes a link to both the official report of the battle and The Life of Major General Zachary Taylor.

The catalyst for the conflict in California was the Bear Flag Revolt, which occurred on June 15, 1846. In the town of Sonoma, American settlers “stormed the home of Colonel Mariano Vallejo, commander of the northern military district in Mexican California, demanding his surrender.”[8] These settlers then declared California to be an independent republic and raised a flag featuring a bear. The place marker refers to a transcript of the speech given after the Bear Flag was raised, as well as the letters of John C. Fremont. Although this revolt was not violent, it was successful in taking control of the territory away from Mexico. This was also significant because American settlers in California declared their independence from Mexico prior to armed conflict, essentially starting the battle for California.

California was further claimed through the Siege of Los Angeles on August 13, 1846. Although this was not a violent encounter, after rounding up Californian soldiers and running the Californian people out of town, Stockton stated that California belonged to the United States.[9] The place marker on the map includes a link to Stockton’s proclamation. This marked the official declaration that the United States wanted to add California to its territory.

The biggest battle of the conflict in California was the Battle of San Pasqual on December 6, 1847. Because the Americans’ gunpowder was wet, “much of this confrontation involved hand-to-hand combat that was favorable to Californios, who were famous for their horsemanship and more equipped with lances and muskets than rifles.”[10] The Americans were defeated at this battle, with eighteen deaths and seventeen injuries compared to the eleven injuries and no deaths suffered by the Californians. Although this battle did not affect the ultimate outcome of the conflict, it was the bloodiest battle for California, with the most loss of life by the Americans, and was surely a set back in the campaign.

The Battle of La Mesa on January 8, 1847, was the final battle in the campaign and was a huge victory for the Americans. The Californians were very outnumbered and suffered large losses, as they “could not withstand the hail of bullets rained upon them.”[11] This battle marked a shift in the war for the United States. Signed after the Battle of La Mesa, the Treaty of Cahuenga officially ended the conflict in California (discussed in the final place mark). The Treaty was more of an agreement to stop fighting than a formal acknowledgement of peace between the two sides. Although the United States did not officially gain California until the Treaty of Guadalupe-Hidalgo in 1848, the end of this conflict marks a significant shift in the war towards American victory.

All of the sources used for this project provide a descriptive narrative of the Mexican-American War, although each author has their own reasons for discussing the war. Lisbeth Haas sets out to provide a complete description of the war “as a military, political, and social event that dynamically shaped the new state and the lives of its Mexican, American, and Indian populations.”[12] In order to do this, Haas focuses primarily on the effect the war had on the native Californians. However, she still provides a relatively unbiased description of events and one can learn a lot about the campaign in California from her article. From the same journal, Mary-Jo Wainwright describes the Bear Flag Revolt in terms of California’s experience with “constant cultural confrontation” by explaining the rebellion’s impact on cultural relations.[13] Douglas Meed’s book, The Mexican War 1846-1848, provides an impartial account of the outbreak of war and the major battles that were fought. This book reads like a textbook and does not seek to make any arguments. Finally, Thomas Robert’s article about the Battle of Resaca de la Palma in The Journal of the American Military History Foundation is understandably focused on the military aspect of the battle. All of these sources serve to provide a different position on the war and can be integrated into one complete picture.

While the Mexican-American War was surely rooted in expansionist policies, Polk’s real motivation for going to war was to annex California before another power did. His administration “publicized, exaggerated, and when necessary provoked foreign threats” in order to convince the American people that going to war with Mexico was necessary.[14] Through these major battles and the eventual Treaty of Guadalupe-Hidalgo, Polk was able to achieve his goal of securing California.


[1] Drew VandeCreek. “Impact.” The Mexican-American War. (accessed October 8, 2011).

[2] Walter A. McDougall. “Expansionism, or Manifest Destiny.” Promised Land, Crusader State: The American Encounter with the World since 1776. Boston: Houghton Mifflin, 1997. 93. Print.

[3] Drew VandeCreek. “Origins.” The Mexican-American War. (accessed October 8, 2011)

[4] Ibid.

[5] “The Northern Campaign.” The US-Mexican War. (accessed October 8, 2011).

[6] Ibid.

[7] Ibid.

[8] Mary-Jo Wainwright. “Milestones in California History: The 1846 Bear Flag Revolt: Early Cultural Conflict in California.” California History 75, no. 2 (1996). October 8, 2011).

[9] Lisbeth Haas. “War in California 1846-1848.” California History 76, no. 2/3 (1997): 342. (accessed October 18, 2011).

[10] Ibid, 344

[11] “Campaign for Los Angeles.” Journal for Negro Education (1949): 69-76. (accessed October 8, 2011).

[12] Haas 333

[13] Wainwright

[14] McDougall 91

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