<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> <P> Once the U.S. declared war on Mexico, Antonio López de Santa Anna wrote to the Mexican government, saying he no longer had aspirations to the presidency, but would eagerly use his military experience to fight off the foreign invasion of Mexico as he had before . President Valentín Gómez Farías, a civilian, was desperate enough to accept the offer and allowed Santa Anna to return . Meanwhile, Santa Anna had secretly been dealing with representatives of the U.S., pledging that if he were allowed back in Mexico through the U.S. naval blockades he would work to sell all contested territory to the United States at a reasonable price . Once back in Mexico at the head of an army, Santa Anna reneged on both agreements . He declared himself president once again and unsuccessfully tried to fight off the U.S. invasion . </P> <Table> Events leading to the American Civil War <Tr> <Td> <Ul> <Li> Slavery </Li> <Li> Northwest Ordinance </Li> <Li> Kentucky and Virginia Resolutions </Li> <Li> Missouri Compromise </Li> <Li> Tariff of 1828 </Li> <Li> Nat Turner's slave rebellion </Li> <Li> Nullification Crisis </Li> <Li> The Amistad </Li> <Li> Prigg v. Pennsylvania </Li> <Li> Texas annexation </Li> <Li> Mexican--American War </Li> <Li> Wilmot Proviso </Li> <Li> Manifest destiny </Li> <Li> Underground Railroad </Li> <Li> Nashville Convention </Li> <Li> Compromise of 1850 </Li> <Li> Fugitive Slave Act of 1850 </Li> <Li> Uncle Tom's Cabin </Li> <Li> Kansas--Nebraska Act </Li> <Li> Ostend Manifesto </Li> <Li> Bleeding Kansas </Li> <Li> Caning of Charles Sumner </Li> <Li> Dred Scott v. Sandford </Li> <Li> The Impending Crisis of the South </Li> <Li> Brown's raid on Harpers Ferry </Li> <Li> 1860 presidential election </Li> <Li> Crittenden Compromise </Li> <Li> Secession of Southern States </Li> <Li> Star of the West </Li> <Li> Corwin Amendment </Li> <Li> Battle of Fort Sumter </Li> </Ul> </Td> </Tr> <Tr> <Td> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> </Td> </Tr> </Table>

Who was president of mexico during the mexican american war