Causes of the Mexican American War of 1846

Battle of Buena Vista during the Mexican-American War, painting by Carl Nebel.

Withholding details relevant to General Zachary Taylor’s conflict with Mexican troops in Texas helped to facilitate Polk’s vision of Manifest Destiny.

The Cabinet meeting of Saturday, May 9th, 1846 began as a discussion of the treatment of special U.S. envoy John Slidell’s mission to Mexico and that nation’s rejection of any negotiations to sell its continental territories to the U.S. Reports received by President Polk later that evening, however, related that Mexican troops had killed and captured several American soldiers under the command of Zachary Taylor. Drafting a war message on Sunday, Polk asserted that war “exists by the act of Mexico herself…” Although both houses of Congress voted for a declaration of war, the evidence of Mexican treachery was false.

General Taylor in Disputed Texas Territory

Zachary Taylor had been ordered into disputed territory in March, erecting fortifications over-looking the Mexican city of Matamoras. The area in question lay between the Rio Grande and the Nueces Rivers, the latter claimed by Mexico as the official boundary. Historians differ as to Polk’s motives for the order; some claim that the President “had not expected or wished the war to begin in this way.” Others suggest that Polk’s order was deliberately designed to provoke a Mexican response, thus enabling him to recommend war.

Cabinet and Congress Respond

Polk’s Cabinet met from 7:30 until 10:00 P.M., unanimous in the resolve to submit a war message to Congress on Monday morning. Secretary of State James Buchanan and Navy Secretary George Bancroft began to prepare, according to Polk, “a history of our causes of complaint against Mexico…” that included correspondence relative to the Slidell mission.

Polk spent Sunday writing his war message, regretting “the necessity…to spend the Sabbath in the manner I have.” Throughout the day key members of Congress spoke with Polk. Not all agreed with the necessity of war. A chief adversary of war was the Democratic Senator from Missouri, Thomas Hart Benton, whose support was needed. Benton, however, voted for the resolution anyway later that week.

Non-Debate in the Congress

A major grievance of the war message came only after the measure was adopted in the ensuing weeks. At issue was whether, according to Polk, “Mexico had passed the boundary of the United States…invaded our territory, and shed American blood upon the American soil.” Because Polk strove for a speedy declaration of war, the accompanying documents were never scrutinized. Actual discussion in the House prior to the vote was only thirty minutes. The opposition party, the Whigs, was victim of a parliamentary gag procedure that disallowed debate or motions.

Although the Senate took longer to approve the measure, it too was not privy to all of the facts surrounding Taylor’s skirmish with Mexican troops in Texas. According to Harvard historian Frederick Merk, Senator John C. Calhoun, who opposed the war, “declared that not 10 percent of Congress would have agreed to the war bill if time had been given to examine the documents.”

Supporting the Troops

In the days following the war declaration, both Democrats and Whigs joined in appropriating funds for the war and reinforcing Taylor. Taylor, commanding 3,900 soldiers, had asked in his dispatches for an additional 5,000 men, but that was before the war resolution.

The Whigs, who would win control of the House in 1846 and control the “power of the purse,” could also rest easy knowing that Taylor as well as the commanding general, Winfield Scott, were Whigs. Success against Mexico would provide a possible presidential candidate in 1848, duplicating the election of 1840 when Whigs elected William Henry Harrison, a war hero from 1811-1812.

The Spirit of Manifest Destiny

James K. Polk began his presidency as an ultra-expansionist. He promised voters in 1844 to solve the Oregon question in America’s favor and to incorporate Mexican-held lands that included New Mexico, California, and the regions in between. When diplomacy failed, war was the only other viable option. The events of March through May in 1846 provided that opportunity.


  1. Bernard DeVoto, The Year of Decision 1846 (Little, Brown and Company, 1943)
  2. Seymour V. Connor & Odie B. Faulk, North America Divided: The Mexican War, 1846-1848 (Oxford University Press, 1971)
  3. Robert W. Johannsen, To the Halls of the Montezumas (Oxford University Press, 1985)
  4. Frederick Merk, History of the Westward Movement (Alfred A. Knopf, 1978)
  5. Polk: The Diary of a President 1845-1849, Allan Nevins, Editor (Longmans, Green and Co., 1952)