Polk shared many of Buchanan 's foreign policy views , and Buchanan was offered the position of Secretary of State in the Polk administration . Though he considered the alternative of serving on the Supreme Court , Buchanan accepted as Secretary of State throughout Polk 's lone term in office . During that time , Polk and Buchanan nearly doubled the territory of the United States through the Oregon Treaty and the Treaty of Guadalupe Hidalgo . In negotiations with Britain over Oregon , Buchanan at first advised a compromise , but later advocated for annexation of the entire territory . Eventually , Buchanan assented to a division at the 49th parallel . After the outbreak of the Mexican – American War , Buchanan advised Polk against taking territory south of the Rio Grande River and New Mexico . However , as the war came to an end , Buchanan argued for the annexation of further territory , annoying Polk , who suspected that Buchanan was primarily concerned with eventually becoming president . Buchanan did quietly seek nomination at the 1848 Democratic National Convention , as Polk had promised to serve only one term , but the nomination went to Senator Lewis Cass of Michigan
1848 Democratic National Convention
1848 Democratic National Convention
James Buchanan