During the Mexican – American War , Butler served as a major general of volunteers . He was General Zachary Taylor 's second-in-command during the Battle of Monterrey and later succeeded Winfield Scott as the commander of American forces occupying Mexico City . The 1848 Democratic National Convention nominated a ticket of Cass and Butler , but the Whig ticket of Taylor and Millard Fillmore won the 1848 presidential election . He attended the Peace Conference of 1861 , which sought to defuse the secession crisis that arose following the 1860 presidential election . During the Civil War , Butler was a War Democrat who favored the Union
1848 Democratic National Convention
1848 Democratic National Convention
William Orlando Butler