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Declarations of war, May 1846

Polk received word of the Thornton Affair, which, added to the Mexican government's rejection of Slidell, Polk believed, constituted a casus belli. Polk said in a message to Congress: "The cup of forbearance had been exhausted even before the recent information from the frontier of the Del Norte. 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. She has proclaimed that hostilities have commenced, and that the two nations are now at war."

The U.S. Congress approved the declaration of war on May 13, 1846, after a few hours of debate, with southern Democrats in strong support. Sixty-seven Whigs voted against the war on a key slavery amendment, but on the final passage only fourteen Whigs voted no, including John Quincy Adams. Later, a freshman Whig Congressman from Illinois, Abraham Lincoln, challenged Polk's assertion that American blood had been shed on American soil, calling it "a bold falsification of history."

Regarding the beginning of the war, Ulysses S. Grant, who had opposed the war but served as an army lieutenant in Taylor's army, claims in his Personal Memoirs (1885) that the main goal of the U.S. Army's advance from Nueces River to the Rio Grande was to provoke the outbreak of war without attacking first, to debilitate any political opposition to the war.

The presence of United States troops on the edge of the disputed territory farthest from the Mexican settlements, was not sufficient to provoke hostilities. We were sent to provoke a fight, but it was essential that Mexico should commence it. It was very doubtful whether Congress would declare war; but if Mexico should attack our troops, the Executive could announce, "Whereas, war exists by the acts of, etc.", and prosecute the contest with vigor. Once initiated there were, but few public men who would have the courage to oppose it.... Mexico showing no willingness to come to the Nueces to drive the invaders from her soil, it became necessary for the "invaders" to approach to within a convenient distance to be struck. Accordingly, preparations were begun for moving the army to the Rio Grande, to a point near Matamoras. It was desirable to occupy a position near the largest centre of population possible to reach, without absolutely invading territory to which we set up no claim whatever.

In Mexico, although Paredes issued a manifesto on May 23, 1846, and a declaration of a defensive war on April 23, both of which are considered by some the de facto start of the war, the Mexican Congress officially declared war on July 7, 1846.