Buchanan studied law at Lancaster, Pa., and in practice there gained a considerable reputation for his wide learning and brilliant oratory. Thus prepared, he went into state politics, then entered the national scene as Representative (1821–31), and was later minister to Russia (1832–33) and Senator (1834–45). A Federalist early in his career, he was later a conservative mainstay of the Democratic party.
He served (1845–49) as Secretary of State under President Polk and, although Polk exercised a strong personal hand in foreign affairs, Buchanan ably seconded his efforts. The quarrel with Great Britain over Oregon was settled peacefully. That with Mexico, which followed the annexation of Texas and the failure of the mission of John Slidell, led to the Mexican War and the Treaty of Guadalupe Hidalgo (1848).
Under President Pierce, Buchanan served (1853–56) as minister to Great Britain. He collaborated with Pierre Soulé, minister to Spain, and John Y. Mason, minister to France, in drawing up the Ostend Manifesto (1854), which was promptly repudiated by the U.S. Dept. of State. His open advocacy of purchasing Cuba (which would presumably have come into the Union as a slaveholding state) won him the hatred of the abolitionists, whom he in turn despised as impractical troublemakers.