<P> During the Battle of Resaca de la Palma the next day, the two sides engaged in fierce hand to hand combat . The U.S. Cavalry managed to capture the Mexican artillery, causing the Mexican side to retreat--a retreat that turned into a rout . Fighting on unfamiliar terrain, his troops fleeing in retreat, Arista found it impossible to rally his forces . Mexican casualties were heavy, and the Mexicans were forced to abandon their artillery and baggage . Fort Brown inflicted additional casualties as the withdrawing troops passed by the fort . Many Mexican soldiers drowned trying to swim across the Rio Grande . Both these engagements were fought before war was declared . </P> <P> In 1846, relations between the two countries had deteriorated considerably and on April 23, 1846, the president of Mexico issued a proclamation, declaring Mexico's intent to fight a "defensive war" against the encroachment of the United States . On April 25, 1846, two thousand Mexican cavalry crossed into the disputed territory and routed a small detachment of American soldiers, sparking the "Thornton Affair". Polk received word of the Thornton Affair, which, added to the Mexican government's rejection of Slidell, Polk believed, constituted a casus belli (cause for war). His message to Congress on May 11, 1846, claimed that "Mexico has passed the boundary of the United States, has invaded our territory and shed American blood upon American soil ." </P> <P> 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 14 Whigs voted no, including Rep. John Quincy Adams . </P> <P> In Mexico, although President 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, Mexico officially declared war by Congress on July 7, 1846 . </P>

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