Complex sentence: It represented a move toward free trade, and was opposed by protectionist elements in the United States, who joined with Americans angry at apparent British support for the Confederate States of America during the American Civil War, ended it in 1866.
Split into simpler sentences: It represented a move toward free trade, and was opposed by protectionist elements in the United States. After the conclusion of the American Civil War, these protectionist elements were joined by Americans angry at apparent British support for the Confederate States of America during the war, and the alliance was successful in terminating the treaty in 1866.