A much more serious dispute in Anglo-American relations arose over the Union capture of Mason and Slidell in November. The two Confederate diplomats ran the blockade on 12 October 1861, sailing from Charleston to Nassau and thence to Cuba. On 7 November they boarded the Trent, a British mail packet, for the remaining leg of their voyage to England. On the next day the USS San Jacinto, under the command of Captain Charles Wilkes, stopped the Trent on the high seas, boarded the vessel, and after a minor skirmish removed Mason and Slidell and their secretaries. Wilkes carried his prisoners to Boston where they were imprisoned in Fort Warren in Boston harbor. Wilkes had acted without instructions and the seizure raised a number of questions of international law that resembled the issue of impressment that had so aroused Americans before 1812. While feigning outrage, the Confederates were delighted with Wilkes's action. They were certain that the Union would never give up the prisoners and that the resulting Anglo-American hostility could only help the Confederate cause overseas. Wilkes was widely applauded in the North, and the seizure of Confederate "traitors" from a British ship struck many as just and a proper response to Britain's assumed partiality toward the Confederacy. The crisis intensified during November and December as Secretary of the Navy Gideon Welles, the House of Representatives, and the Northern press extolled Wilkes's heroism. Meanwhile, the British developed an extensive case for the illegality of his action and made it clear that they regarded the seizure as an affront to Britain's national honor. Prime Minister Palmerston was furious. Britain prepared for war, and sent more than eleven thousand troops to Canada. Russell charged that Wilkes had violated international law and instructed Lyons to demand the immediate release of Mason and Slidell and an apology. At Queen Victoria's request, Prince Albert, although mortally ill, softened Russell's dispatch and provided an out for the Americans by allowing Seward to deny that Wilkes had acted under instruction.
Seward may have initially been pleased by Wilkes's action, but he very quickly adopted a moderate tone in a dispatch to Adams that the latter presented to Russell. Seward's note did much to defuse the crisis at the highest level. Lyons also presented Russell's demands in such a way to make a favorable American response more likely. Finally, on 25 December Seward responded by informing the British that Wilkes had indeed acted on his own and, while not violating international law, had made certain technical errors. The two diplomats would be "cheerfully liberated" and turned over to Lord Lyons. Seward had convinced Lincoln, others in the cabinet, and a number of prominent senators that retaining the Confederates was decidedly not in the Union's interest.
The furor generated by the Trent affair dissipated quickly. Support for Wilkes disappeared as American attention turned to the war at home and journals published calculations of the costs of a war with Britain. Seward had demonstrated genuine statesmanship that Russell and others recognized. For the first time, British officials began to reconsider their early estimation of the secretary of state. Seward perhaps understood better than before that threatening war was, indeed, a dangerous policy. Seward's reputation among influential associates, however, did not improve. A number of prominent Radical Republicans, never supportive, renewed efforts to remove him from office. By this time Lincoln had come to regard Seward as an effective secretary and as a valuable ally in his opposition to the Radical Republicans and was determined to keep him in the cabinet.
Seward meanwhile sought to capitalize on the relief felt in Britain and the Union over the peaceful resolution of the issue. Although the details remain vague, there is evidence that he facilitated the deployment of British troops in Canada—they had arrived after the St. Lawrence River froze over. Whatever the case, Seward skillfully used the story that he had granted permission to Her Majesty's troops to cross American territory to emphasize that he held no animosity nor aggressive intent toward Britain. Seward also resisted suggestions that the United States abrogate or let expire the Marcy-Elgin Reciprocity Treaty of 1854. The treaty, set to expire in 1864, eliminated duties in trade between the United States and Canada, opened the St. Lawrence to the United States, and regulated fishing off the Canadian coast. Seward saw Canadian-American free trade as a means of integrating the Canadian economy into America's as a prelude to annexation.
Seward also concluded negotiations with Lyons in April 1862 for the suppression of the African slave trade. The agreement extended the reciprocal right to search and detain merchant ships off the coasts of Africa and Cuba and established prize courts in Sierra Leone, the Cape of Good Hope, and New York. In February 1863, Seward and Lyons expanded the agreement to include the coasts of Madagascar, Puerto Rico, and Santo Domingo. These treaties completed efforts to end the Atlantic slave trade that had begun a half century earlier. (In the absence of Southern senators, Seward also recognized Liberia and Haiti in 1862 and treaties of amity and commerce with Liberia on 21 October 1862 and Haiti on 3 November 1864.) These agreements with Britain after the Trent crisis contributed to the restoration of friend-lier relations. This was especially important at a time when pressure for mediation was increasing in Britain and France because of a developing crisis in cotton textile manufacturing centers.