The American Civil War significantly worsened British-American relations.

At first, there was some sympathy for the Confederacy's cause because of Britain's economic ties with them, especially through the cotton trade. But British support for the Confederacy declined as the war went on and the Union took a firm stance against slavery. Events like the Trent Affair, in which the U.S. Navy intercepted a British ship transporting Confederate diplomats, contributed to the escalation of tensions. This incident nearly led to war between the two countries, underscoring how shaky their relationship was at this time. Furthermore, the public's opposition to slavery and the Union's successful diplomacy made Britain's position even more difficult. In the end, the Civil War revealed and widened preexisting rifts, having a long-lasting effect on US-British diplomatic ties. The conclusion of the conflict had mostly negative immediate impacts, but in the long run, it led to a more normalized relationship.