- Preserving the Union: Buchanan sought to maintain the unity of the country amid growing sectional tensions over the issue of slavery. He attempted to mediate between the North and the South, but his efforts were unsuccessful, and the country eventually descended into the Civil War.
- Gadsden Purchase: Buchanan oversaw the acquisition of the Gadsden Purchase from Mexico in 1853. This purchase added territory to the United States that included present-day southern Arizona and New Mexico.
- Japan Trade Treaty: Buchanan negotiated the Treaty of Kanagawa with Japan in 1854, which established diplomatic relations and opened Japanese ports to American trade.
- Ostend Manifesto: Buchanan was involved in the drafting of the Ostend Manifesto, which called for the annexation of Cuba and criticized European influence in the Americas. The manifesto was controversial and did not lead to any concrete actions.
- Dred Scott Case: Buchanan supported the Supreme Court's decision in the Dred Scott case, which ruled that African Americans were not citizens and could not sue in federal court. This decision further inflamed sectional tensions and contributed to the growing division between the North and the South.