“After a long-continued series of menaces [Mexicans] have at last invaded our territory and shed the blood of our fellow-citizens on our own soil,” President James K. Polk thundered on May 11, 1846, in his request that the U.S. Congress declare war against Mexico. He’d been preparing such a request days earlier, however, even before that initial conflict, and in fact, some of his contemporary political opponents – and some scholars today – believe that Polk intentionally provoked the war as part of a land grab.

The Buildup to War: Tensions Rising

After Texas won its independence from Mexico in 1836, the United States resisted calls to add what became the Lone Star State to its ranks because Mexico, which did not recognize Texas’s sovereignty, had threatened to declare war if the U.S. did so. In 1844, President John Tyler sought to annex Texas, but the Senate rejected his efforts both because of the Mexican threat and because doing so would throw off the tenuous balance of slave and free states. He tried again shortly before leaving office the next year – with the support of President-elect Polk – and this time succeeded. Texas was officially admitted to the union on Dec. 29, 1845. Mexico did not declare war.

Polk’s Negotiations

Relations between Mexico and the U.S. remained tense, however. Polk dispatched a negotiator, John Slidell of Louisiana, to Mexico to negotiate the Texas border – and more importantly to purchase California and New Mexico for $30 million. The newly elected Mexican president, José Joaquín Herrera, was aware of the Americans’ intentions to acquire his country’s land and, worried that negotiation would lead to internal turmoil in his country, refused to see Slidell. A few days later he was deposed by a military general, who also declined to negotiate with Slidell.

Declaration of War

Angered, Polk ordered troops led by Gen. Zachary Taylor to occupy the disputed Texas territory. On May 9, 1846, he began drafting a declaration of war, arguing that the Mexican government’s refusal to settle the Texas question left him no choice. But that night, he learned that on April 25, Mexican troops had attacked Taylor’s forces, killing and injuring sixteen. This fact was prevalent in his speech to Congress on May 11: “The Mexican Government not only refused to receive [Slidell] or listen to his propositions, but after a long-continued series of menaces have at last invaded our territory and shed the blood of our fellow-citizens on our own soil.” Moreover, he continued, “Not only was the offer rejected, but the indignity of its rejection was enhanced by the manifest breach of faith in refusing to admit the envoy who came because they had bound themselves to receive him.” Congress approved the declaration two days later.

A Divided Country

While Congress overwhelmingly approved the declaration, the war itself was controversial. Anti-slavery advocates argued that Polk, a Democrat, was acquiring this land to further Southern slaveholders’ interests. His political opponents, the Whigs, claimed that he had provoked the Mexican attack in a land grab. Whatever Polk’s motives, in the end he got what he wanted: In the 1848 Treaty of Guadalupe Hildago the U.S. purchased from the defeated Mexico land that today comprises New Mexico, Utah, Nevada, Arizona, California, Texas and western Colorado -- all for $15 million.