Trent Affair, incident during the American Civil War that severely tested diplomatic relations between the United States and Britain. On November 8, 1861, Captain Charles Wilkes of the U.S. vessel San Jacinto intercepted at sea the British mail steamer Trent, bound for Europe from Havana, Cuba. He took from the ship two Confederate commissioners who were among the passengers, James Mason, who was accredited to Britain, and John Slidell, who was accredited to France. The two diplomats were subsequently held as prisoners in Boston, but Britain demanded their release on the ground that they had been forcibly taken from a neutral vessel on the high seas upon a voyage from one neutral point to another, and that therefore Wilkes's action had been illegal. Wilkes had been hailed as a hero in the U.S., and the possibility of war between the two countries seemed imminent. On December 26, however, U.S. Secretary of State William Henry Seward repudiated the capture of the prisoners, who were released the following January.