Marching on Mexico City

By the spring of 1847, with the Mexican-American War seeming to stagnate after a year of bitter fighting, the U.S. military had invaded and occupied all of northern Mexico while blockading every major enemy port. Unable to take back its northern provinces or overcome internal political turmoil, the Mexican government refused to seek terms even as it suffered defeat after defeat. The Polk Administration, growing impatient in Washington, D.C., consequently made the seminal decision to compel capitulation with a dramatic amphibious invasion of Central Mexico. Driven by a complicated mixture of shifting politics, expansionist policies, and military strategy that cumulatively encouraged aggressive coercion, the presidential directive set conditions for a decisive march on the Mexican capital and the attainment of all American strategic objectives.[1]

The idea of Manifest Destiny emerged as a nationalistic propellent for the United States and its settler communities to aggrandize territories west of the Appalachian Mountains following the Louisiana Purchase of 1803. Famously described by journalist John O’Sullivan in 1845 as an imperative for the American people to, “overspread the continent allotted by Providence for the free development of our yearly multiplying millions,” the mixture of spiritual, racial, economic, and geopolitical motivations centered on expanding US dominance across the Great Plains and to the shores of the Pacific Ocean.[4] While Jacksonian populism and sustained frontier conflict added militancy to the fervor, the acerbic debate over the prospect expanding chattel slavery into western acquisitions and moral objections to provoking wars of aggression would serve as political checks on realizing the full scope of the continental dream.[5]

The question of formal and sanctioned western expansion became acute with the U.S. annexation of the Republic of Texas and its contested territorial claims in December of 1845. While Texas itself had aimed to conquer a “vast extent of territory, stretching from the Sabine to the Pacific,” as announced by its second president, Mirabeau Lamar, at his inauguration in 1838, the United States’ willingness to enforce the Lone Star Republic’s controversial claims made a war with Mexico a near-certain outcome.[6] In particular, the Texans’ assertion that their southern boundary extended to the Rio Grande, as opposed to the Nueces River farther north, and the Polk administration’s sudden military occupation of the contested space in April of 1846, set the stage for a war that would decide the fate of the middle expanse of the North American continent.

This disagreement over Texas found both the United States and Mexico ill-prepared to project and sustain substantial military forces across distant fronts. While the U.S. Army started with a pre-war authorization of approximately 7,000 regulars drawn from scattered frontier garrisons, the Mexican Army fielded a larger, though less professional, force of 19,000 men with ability to rapidly double in size through conscription.[7] In terms of economic capacity, the Americans, with a much larger population and industrial base, held advantages in production and naval warfare, while the Mexicans plausibly could maximize the advantages of defensive tactics, an experienced officer corps, and the motivations of nationalistic resistance. The U.S. Navy’s dominance along both coastlines, in particular, would prove instrumental in the conquest of California and provide Polk a valuable asymmetric advantage when contemplating options to expand the war.[8] 

A complicated constellation of political factors both galvanized and restrained the Polk administration’s initial decisions to provoke a war. The election of 1844, which had pivoted on the question of territorial expansion, saw Polk and the Democrat Party win a decisive governing mandate with majorities in both the Senate and the House.[9] With the platform from the Democrat Convention stating that, “the reoccupation of Oregon and the re-annexation of Texas” were “great American measures,” the new administration felt empowered to conquer or purchase the two territories through any means possible.[10] However, while disagreements over slavery expansion and the morality of provoking a controversial war threatened Democrat cohesion, the Whigs, as the opposition party, opposed westward expansion and dominated the senior Army officer corps.[11]

In this political climate, the new administration set an unambiguous national policy of immediate territorial expansion at the expense of Mexico and the British Empire. In his inaugural address, Polk announced that the United States would “enlarge its limits” and “extend the dominions of peace over additional territories” through “peaceful acquisition.”[12] While the president had attempted to purchase lands from Mexico and kept his public assertions limited to acquiring Texas under the pretext of pre-existing claims, he informed his cabinet that his actual “purpose” was to attain “California, New Mexico, and perhaps some other of the Northern Provinces of Mexico.”[13] This unannounced policy extension would inform decisions to divide scarce military resources between the Texas theater and additional objectives in Santa Fe, Monterey, and San Francisco. 

A powerful combination of party consensus and aggressive policy aims drove the adoption of an ambitious continental military strategy with cooperative land and maritime campaigns. Relying on controversial accusations that Mexico had “shed American blood upon American soil” when it attacked U.S. forces sent to occupy the disputed territory along the north bank of the Rio Grande on April 25, 1846, Polk quickly attained bi-partisan congressional authorization for war and ordered a series of ground invasions to secure Texas, New Mexico, and California.[14] Simultaneously, he ordered a naval blockade along both the Gulf and Pacific coastlines to inflict economic privation and safeguard sea lines of communication. This resulted in a strategy of distributed pressure that established possession of the coveted western territories while attempting to coerce the proud Mexican government to agree to a financial settlement.[15]

Despite the U.S. military’s success in the northern provinces, Mexico City refused to negotiate as the war stagnated into the spring of 1847. During this period the Polk Administration suffered a major political defeat in the 1846 Congressional Elections where the Whigs won majority control of the House due to localized frustrations with unpopular tariffs, slavery disagreements, and, in some areas, anti-war sentiment. While the public still broadly embraced the spirit of Manifest Destiny and Democrats yet retained control of the senate, Polk felt an increased urgency to conclude the war with a clear victory in order to better position his party for the 1848 presidential election.[16] Complicating the political situation further, the administration would be forced to rely on Whig-affiliated officers such as the U.S. Army’s commanding general and presidential aspirant, Winfield Scott, to plan, resource, and implement any significant change in the military strategy.[17]

Throughout this period of political transition, the Polk Administration maintained its policy of defending the Texas annexation while indicating intent to permanently occupy New Mexico and California. In his second annual message to the Congress in December of 1846, Polk stated that the war would be, “prosecuted with vigor as the best means of securing peace” while emphasizing that it would be “proper to provide for the maintenance of civil order and the rights of the inhabitants” of the “important conquests.”[18] As the war dragged on longer than expected, the administration funded and mobilized 50,000 volunteer soldiers to replace losses, assist with pacification, and free regular army regiments for offensive operations.[19] However, with the Mexicans still refusing to negotiate due to nationalistic pride and inability to reach internal consensus, the policy would require an altered strategy to accommodate the new political paradigm.

This intersection of political urgency and policy commitment led to a fateful decision in the final weeks of 1846 to expand the current military strategy with a new offensive into the heart of Mexico. With the Army consolidating gains in the north and the Navy blockading both coasts, a small field army under Winfield Scott would conduct an amphibious assault against the Atlantic fortress of Veracruz, advance into the interior, and, if need be, seize Mexico City.[20] The decision, which found unanimous support with Polk’s cabinet secretaries only after several heated debates, represented a willingness to accept elevated political risk to achieve greater coercive influence against the Mexican leadership.[21] While Polk only reluctantly appointed the Whig general to command due political tensions, he recognized that the complicated operation required a veteran commander who could plan the joint endeavor and manage service rivalries.[22]

The decision to march on the Mexican capital proved both successful and frustrating for the American leadership. Though Scott and his 11,000 soldiers overcame fierce resistance to capture Veracruz, win at Cerro Gordo, and eventually take Mexico City by storm in September 1847—a feat that the Duke of Wellington in England called “unsurpassed in military annals”—the Mexican government still refused to concede and appeared ready to endure indefinite occupation. The resulting financial burden and mobilization of more volunteers to combat a determined guerilla resistance by elusive Mexican mounted forces further strained the administration’s war policy as a faction of outraged Democrats began to call for an “All of Mexico” annexation.[23] However, the Mexican leadership, worried about their eroding legitimacy and Mayan revolts in the Yucatan, finally capitulated in the spring of 1848 and sold New Mexico and California under the terms of the Treaty of Guadalupe Hildago for $15 million.[24]

The American decision to invade Central Mexico remains an instructive example for future strategists to, as required by modern U.S. joint doctrine, “constantly evaluate effects and objectives, align them with strategic objectives, and verify that they are still relevant and feasible.”[25] While the Polk administration could have simply defended its northern gains in the hopes of an eventual settlement, rising tension between political urgencies and resolute war aims demanded a dramatic alteration in their strategic approach. These decisions stemmed from a complicated mix of party politics, overt and covert policy formulation, and strategic optionality that resulted in successful coercive tactics. This suggests that strategists in the 21^st century, even as they negotiate unpredictable challenges, must be similarly responsive to changing political calculus or risk policy atrophy and inability to achieve strategic objectives.

Critics, however, may argue that the Polk Administration’s decision-making was actually less central to the outcome of the war than more positive studies may suggest. Instead, a minority of scholars, such as historian Irving Levinson, have placed the emphasis on Mexican disunity and native revolts, rather than American strength and strategy, to explain U.S. battle success, the staying-power of the occupations, or the totality of the final surrender.[26] This explanation, though supported by evidence of wide-spread indigenous uprisings across Mexico throughout the war, actually compliments the orientation and timing of Polk’s decision to risk an amphibious assault on Veracruz and assault on the Mexican capital. It suggests that the American president’s decision, even as he balanced fluctuations in domestic affairs, created additional dilemmas that eventually compelled the Mexican leadership to acquiesce and surrender.

These outcomes hold insights for modern strategists as they apply strategic art amidst a global environment that is once again being defined by great power rivalry. Recognizing Clausewitz’s dictum that, “the degree of force that must be used against the enemy depends on the scale of the political demands on either side,” it means that, similar to Polk’s multi-faceted challenges in 1847, strategic thinkers must carefully consider how dynamic politics, expanding policies, tailored strategies, and the vagaries of battlefield chance will drive executive decision-making within acceptable risk parameters.[27] If American leaders during the Mexican War demonstrated the value of understanding how to make impactful decisions within evolving political-military settings, future strategists, in their own time and place, will likewise be required to devise creative ways to attain strategic success and avoid campaign stagnation.

[1] K. Jack Bauer, The Mexican War, 1846–1848 (Lincoln: University of Nebraska Press, 1992), 392-394.

[2] Carl von Clausewitz, On War, edited and translated by Michael Howard and Peter Paret (Princeton University Press, 1984), 584.

[3] Benjamin Swenson, “Annexation as War”: The 1844 Presidential Election and US-Mexican Conflict,” War History Network, January 15, 2025.

[4] John O’Sullivan, “Annexation,” United States Magazine and Democratic Review, vol xvii (1845): 5-6.

[5] Amy Greenberg, A Wicked War: Polk, Clay, Lincoln, and the 1846 U.S. Invasion of Mexico (New York: Alfred A. Knopf, 2012), 14-15

[6] Mirabeau B. Lamar’s Inaugural Address, Houston, Dec. 10, 1838, in The Papers of Mirabeau Buonaparte Lamar,1798-1859, ed. Charles Adams Gulick, Jr. (Austin: A. C. Baldwin, 1922), 2: 320-321.

[7] Stephen A. Carney, Guns Along the Rio Grande: Palo Alto and Resaca de la Palma, CMH Pub. 73-2 (Washington, DC: U.S. Army Center of Military History, 2005), 6.

[8] Allan Milett and Peter Maslowski, For the Common Defense: A Military History of the United States (The Free Press, 1984, reis., 1994), 151, 155.

[9] Charles Sellers, James K. Polk: Continentalist (Princeton: Princeton University Press, 1966), 310-311.

[10] 1844 Democratic Party Platform, May 27, 1844, The American Presidency Project, University of California at Santa Barbara. Accessed on January 26, 2025.

[11] Robert Merry, A Country of Vast Designs: James K. Polk, The Mexican War, and the Conquest of the American Continent (New York: Simon & Schuster, 2009), 288-289.

[12] James K Polk Inaugural Address, March 4, 1845, The American Presidency Project, University of California at Santa Barbara. Accessed on January 26, 2025.

[13] James K. Polk, Polk: The Diary of a President, 1845-1846, edited by Allan Nevins (New York: Longmans, Green and Co., 1929), 106.

[14] James K Polk Special Message to Congress on Mexican Relations, March 4, 1845, The American Presidency Project, University of California at Santa Barbara. Accessed on January 28, 2025.

[15] Bauer, The Mexican War, 1846–1848, 67-68, 394-395.

[16] Merry, A Country of Vast Designs, 315-316; Brian Walton, “The Elections for the Thirtieth Congress and the Presidential Candidacy of Zachary Taylor,” The Journal of Southern History 35, no 2 (1969): 187-188.

[17] Sellers, James K. Polk: Continentalist, 438-439.

[18] James K. Polk’s Second Annual Message, December 8, 1846, The American Presidency Project, University of California at Santa Barbara. Accessed on January 26, 2025.

[19] Bauer, The Mexican War, 69-70.

[20] Timothy D. Johnson, A Gallant Little Army: The Mexico City Campaign (Lawrence: University of Kansas Press, 2007), 10-11.

[21] Merry, A Country of Vast Designs, 319.

[22] Johnson, Gallant Little Army, 15-16.

[23] Benjamin Swenson, The Dawn of Guerrilla Warfare: Why the Tactics of Insurgents against Napoleon failed in the US Mexican War (Yorkshire and Philadelphia: Pen and Sword Books, 2023), 168-170.

[24] Wellington quoted in Milett and Maslowski, For the Common Defense, 156-157.

[25] Joint Chiefs of Staff, JP 5-0 Joint Planning (Washington, D.C.: Joint Chiefs of Staff, 2020), I-2.

[26] Irving W. Levinson, Wars Within War: Mexican Guerrillas, Domestic Elites, and the United States of America, 1846–1848 (Fort Worth: Texas Christian University Press, 2005), 113-114.

[27] Clausewitz, On War, 585.