The two-year war (1846–48) was fought over the United States’ annexation of Texas. The events that led up to the conflict began in 1837 when President Andrew Jackson (1767–1845) recognized Texas as independent (this was just after Texas had won its war with Mexico). Republic of Texas president Sam Houston (1793–1863) felt that protection against a Mexican invasion may be necessary, so he eyed annexation to the United States. In the meantime, Mexican president Antonio Lopez de Santa Anna (1794–1876) warned that such an action on the part of the United States would be “equivalent to a declaration of war against the Mexican Republic.” In June 1844 the U.S. Senate rejected a proposed annexation treaty. But later that year Democratic Party nominee James K. Polk (1795–1849), an ardent expansionist, was elected president. Because the annexation of Texas had figured prominently in his campaign platform, outgoing President John Tyler (1790–1862) viewed Polk’s victory as a public mandate for annexation, and he recommended that Congress pass a joint resolution to invite Texas into the union. Congress did so in February, and President Tyler signed the resolution on March 1, 1845, three days before leaving office.

Mexico responded by breaking off diplomatic relations with the United States. A border dispute made the situation increasingly tenuous: Texas claimed that its southern border was the Rio Grande River, while Mexico insisted it was the Nueces River, situated farther north. In June President Polk ordered Brigadier General Zachary Taylor (1784–1850) to move his forces into the disputed area. In November the U.S. government received word that Mexico was prepared to talk. Polk dispatched Congressman John Slidell (1793–1871) to Mexico to discuss three other outstanding issues: the purchase of California (for $25 million), the purchase of New Mexico (for $5 million), and the payment of damages to American nationals for losses incurred in Mexican revolutions. This last point was critical to the negotiations, as Polk was prepared to have the United States assume payment of damages to its own citizens in exchange for Mexico’s recognition of the Rio Grande as the southern border of Texas.

But upon arrival in Mexico City, Slidell was refused the meeting—President José Joaquín Herrera (1792–1854) had bowed to pressure, opposing discussions with the United States. When Polk received news of the scuttled talks, he authorized General Taylor to advance through the disputed territory to the Rio Grande. Meanwhile, Mexico overthrew President Herrera, putting into office the fervent nationalist General Mariano Paredes y Arrillaga (1797–1849), who reaffirmed Mexico’s claim to Texas and pledged to defend Mexican territory.

While Polk worked through Slidell to get an audience with the Mexican government, the attempts failed, and on May 9 the Cabinet met and approved the president’s recommendation to ask Congress to declare war. The next day news arrived in Washington that on April 25 a sizeable Mexican force had crossed the Rio Grande and surrounded a smaller American reconnaissance party. Eleven Americans were killed and the rest were wounded or captured. On May 11 Polk delivered a message to Congress, concluding, “Mexico has … shed American blood upon the American soil…. War exists … by the act of Mexico herself.” By the time the war was officially declared on May 13, just more than one year after Polk had been sworn into office, General Taylor had already fought and won key battles against the Mexicans and had occupied the northern Mexico city of Matamoros.