As the crow flew, about 1,000 miles separated Zachary Taylor’s forces along the Rio Grande and Washington, D.C.
Waiting for word in Washington City, President James Polk grew impatient. A staunch expansionist, Polk ardently believed in the concept of Manifest Destiny—that it was the United States’ fate to control all of the American continent from ocean to ocean. He had tried to convince Mexico to sell land to the United States, but that failed miserably when the American envoy was denied credentials. Now Polk stewed in Washington, looking for ways to get at Mexico.
Polk’s mission was aided on May 8, 1846 (the same day that 1,000 miles away, Taylor fought the Battle of Palo Alto), with the arrival of John Slidell. Having been sent to Mexico, it had been Slidell who was unceremoniously denied by Mexican leadership and sent packing the previous winter. Just as he had written the past December, Slidell now personally advocated that war was now the only remaining course left to the United States, and, as Polk wrote in his diary, “In this I agreed with him, and told him it was only a matter of time when I would make a communication to Congress on the subject, and that I had made up my mind to do so very soon.”
The next day (while Taylor defeated the Mexicans at Resaca de la Palma), Polk convened his cabinet members. Before them, Polk suggested “in my opinion” that because of Slidell’s treatment and the standoff along the Rio Grande, the United States had “ample cause of war,” and looked for feedback. Regardless of Polk’s chomping at the bit, though, did the U.S. really have enough reason to declare war on Mexico? The cabinet went back and forth, and Secretary of State (and future president) James Buchanan “said he would feel better satisfied in his course if the Mexican forces had or should commit any act of hostility,” but the cabinet’s consensus, overall, was that Polk could proceed.
Then, that evening, Polk received word that eradicated any hesitation. He quickly re-convened the cabinet and told them what had happened. Taylor’s dispatches regarding Cpt. Seth Thornton’s ambush on April 25 filled the cabinet’s want for signs of Mexican aggression completely. Here then, was something that Polk could go to Congress with.
Over the next two days Polk worked on his message to Congress before, on May 11, it made its way to Capitol Hill. Polk’s message to Congress went on for nearly six pages (and can be read here), in which he spent the most time re-telling John Slidell’s voyage to Mexico and repulse. But then, coming to the military actions along the Rio Grande, Polk penned arguably the most-famous line of the entire proclamation: “But now, after reiterated menaces, Mexico has passed the boundary of the United States, has invaded our territory and shed American blood upon the American soil.” Polk declared that Mexico “has proclaimed that hostilities have commenced, and that the two nations are now at war.”
Polk’s proclamation harkened back to one of the main issues surrounding the entire conflict—where exactly was the “boundary” of the two nations? After their revolution Texans always insisted on the Rio Grande, and after Texas’ annexation in 1845, that cry was taken up by Americans as well. But the Mexicans always insisted, and still did, that the Nueces River was the true dividing line. Also, regardless of Polk’s message that Mexico had “proclaimed that hostilities have commenced,” such was not the case. It would not be until July 7, a little less than two months away, before Mexico finally resolved, in phrasing similar to Polk’s, that Mexico needed to go to war “in use of the natural defence of the nation, will repel the aggression which has been initiated and is sustained by the United States of America, against the Mexican Republic, having invaded and committed hostilities in various Departments of its territory[.]”
On Capitol Hill, both the House of Representatives and the Senate began to respond to Polk’s request for a declaration of war. In both the House and Senate, while the answer to Polk’s request was largely in support of granting it, opposition soon reared its head. Within the House, Congressman and former president John Quincy Adams led a contingent of northern representatives who saw Polk’s request for war as little more than an excuse to expand land and thus, enlarge the power of slave owners.
One of Adams’ compatriots in arguing against the war, Joshua Giddings from Ohio, said, “Sir, no man regards this war as just. We know, the country knows, and the civilized world are conscious that it has resulted from a desire to extend and sustain an institution on which the curse of the Almighty most visibly rests.” But the anti-war crowd’s 14 votes against Polk’s proclamation were washed away by the 174 Congressmen who voted in favor of the conflict. In the Senate, similar anti-war speeches were likewise defeated, and a final vote, held on May 13, 1846, came to 42 in favor, 2 opposed, and 3 abstentions.
The same day as the Senate’s vote, 170 years ago, “About 1 O’Clock P.M. a committee of Congress waited on me and presented the act declaring War against Mexico,” Polk wrote in his diary. “I read it in their presence & approved and signed it.”
James K. Polk had his war.
 James K. Polk, The Diary of James K. Polk, Vol. 1., (Chicago: A.C. McClurg & Co., 1910), 382.
 Polk 384-85.
 Amy Greenberg, A Wicked War: Polk, Clay, Lincoln, and the 1846 U.S. Invasion of Mexico (New York: Alfred A. Knopf, 2012), 105-106.
 K. Jack Bauer, The Mexican War: 1846-1848 (Lincoln: University of Nebraska Press, 1974),68-69.
 Polk, 395.