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.

Buchanan was nominated as a Democratic candidate for the presidency in 1856, with John C. Breckinridge as his running mate, and he won the election over John C. Frémont, the candidate of the newly formed Republican party, and Millard Fillmore, candidate of the Whig and Know-Nothing parties. Buchanan did not have the majority of the popular vote, and his moderate views were disliked and mistrusted by extremists both in the North and in the South.

Although he attempted to keep the "sacred balance" between proslavery and antislavery factions, in his administration the United States plunged toward the armed strife of the Civil War. Buchanan, who disapproved of slavery as morally wrong, felt that under the Constitution slavery had to be protected where it was established and that the inhabitants of a new territory should decide whether that territory should be free or slave. He angered many in the North by renewing efforts to purchase Cuba and by favoring the proslavery Lecompton Constitution in Kansas.

As his administration drew to a close, after the election (1860) of Abraham Lincoln to succeed him as President, Buchanan was faced with the secession of the Southern states. Very learned in constitutional law, he maintained that no state had the right to secede, but he held, on the other hand, that he had no power to coerce the erring states. He believed that the federal government was authorized to use force only in protecting federal property and in collecting customs. Therefore the question of the federal forts in Southern states became of great importance, particularly in South Carolina.

Buchanan tried desperately to keep peace and promised South Carolina congressmen that no hostile moves would be made as long as negotiations were in progress. When Major Robert Anderson moved U.S. troops from Fort Moultrie to Fort Sumter, there was an outcry from South Carolina that the President's promise had been broken. Buchanan defended Anderson but, reluctant to act, sent supplies to Fort Sumter only belatedly. He was battered with criticism from North and South, and shortly after his administration ended, gunfire at Fort Sumter precipitated the war. John Bassett Moore edited his works (12 vol., 1909-11).